Traditional advice for making megadungeons in older versions of D&D is in addition to any rooms with Monsters, Treasure, Traps, or "Tricks", there should also be at least 50 to 60 percent "Empty" rooms, which contain nothing overtly threatening or valuable. Now, there's several arguments for including these empty rooms that I buy, so I'm not going to accept any answer which primarily says, "Don't include empty rooms". The main issue I run into with empty rooms, however, is that they're boring in the way that I've been including them. They don't do their job of increasing tension, and the set dressing included around them hasn't been sufficiently interesting to my players either. My question is this: How can I make empty rooms interesting, by increasing tension or simply being interesting in and of themselves?
By definition, nothing's going to happen in an empty room (though see below). There are no hidden doors to find, no puzzles to solve, no enemies to fight. So what's their purpose?
Bringing the dungeon to life
While all the orcs may sit around in a guard room waiting for PCs to show up, where do they sleep, what do they eat, what happens to their trash? Think of a dungeon not just as a place to have fights in, but as an actual place that serves a purpose. Once you do, the dungeon will start making sense. "Empty" rooms will increase immersion for the players, and force you to consider things you may not have otherwise. Should there be a pet otyugh that eats the trash? Are the orcs demanding cows from the local farmers for "protection"? Do they keep a famed halfling cook enslaved, to make them rare steaks?
Once the dungeon makes sense, you can use that to place clues for things that dwell deeper within. Has a horde of zombies marched through to guard the necromancer's abode within, and left a few rotten body parts here and there? Has a malevolent force been summoned and spoiled all the meat and milk in the dungeon? Is there a golem maintenance station, with oil to grease them, rags to polish them, and a mad goblin mechanic? These may be vital clues PCs need to prepare for a tough fight, or they can simply let them form theories and feel smart about being right once they do come upon a golem or three.
These rooms are empty now, but they don't have to stay that way. PCs may choose to fortify a room and bring the enemy to them - such tactical choices could be an important part of dungeoncrawling. Orcs may chase PCs through the dungeon, or randomly come across them in a kitchen because they were feeling peckish.
Note that some of these ideas would introduce creatures or things into rooms that PCs may choose to interact with, which would make them non-empty. In a way that's fine: you started with an empty room but found something interesting to put inside after all. If that's not your intention, well, don't put goblins or halflings or otyughs in. Stick to body parts and greasy pots and shadowy portraits and bloodstains and scent of brimstone and fresh flowers and broken mirrors and...
Tell a story with your empty rooms, or offer tactical choices. Preferably both.
Do you know the room is empty? (Of course you do.) Do your players know? How do they know?
Is the room totally smooth material without a single crack or joint? That would be most unusual, and hence interesting. Dungeons are typically uneven and roughly hewn, run-down by poor climate, and probably not entirely clean. Is there as much as a piece of rotting root, or a torch, or a lantern hanging from a chain? However uninteresting, there will almost certainly be something that will catch a bored (or shrewd) eye! Maybe the lantern is swaying ever so gently - from recent use, or simply a draft? Who knows! Dun-dun-dunn!
If empty rooms have more than two exits (heck, more than one) you can make it interesting or necessary for the party to leave navigation markers.
You might also highlight what's not in this room -- perhaps finally a refuge from the all-pervasive stink in this area, or you're finally far enough away from the [forge/waterfall/torture chamber] to no longer hear the noise from there.
Try "The room appears to be empty." This subtle clue may lead your party to look for things, finding nothing. Then they will know it is indeed empty.
Empty of threat, but there can be plenty of other things to make it interesting:
- Maybe it's just a smell (pleasant or not)
- the corpse/bones of a past adventurer (looted)
- some small inconsequential animals that flee (spiders or normal sized rats)
There could also be something potentially useful:
- Maybe there's a pool of water that could be made potable with the appropriate spell.
- A wall full of edible fungus (assuming the PCs figure out it's edible).
- A spyhole to another part of the dungeon!
- The mechanism for a trap. This could reset a trap they've already sprung, or disable a trap in a room they've yet to visit!
- An ancient altar to a god (religion check to pray and get some kind of blessing/healing)
- Abandoned monster workshop (no wonder their weapons are so bad!)
- A jail cell with a prisoner (might help, might flee if freed)
- Non-portable treasure (life size statues, huge paintings, a massive pile of copper pieces fused by a previous fireball spell)
Make a list of room types for the kind of area you are mapping, for example barracks, storeroom, larder, kitchen. For ruins, you can go by what the ruins used to be. These can be in a greater state of decay, such as an armory with the rusted remains of swords and axes.
Empty rooms will not appear empty. They will have a room type from your list and improvised or pre-detailed contents appropriate for that room type. So will the occupied rooms. Thus, to search for secret doors or treasure, the players must examine the contents of the room. Each empty room adds to tension, because they never know what is hidden in the room (treasure, secret door, trap or monster) or which rooms to skip.
Also, remember to make restocking rolls for all empty rooms and cleared rooms when the party returns to a dungeon. Just because a room was empty last time doesn't mean it's still a safe place to camp.
Potential Tactical Space
If you're playing a game with lots of empty dungeon rooms, then you should consider all of those potential combat zones. And you should consider making them have tactical value in some way that maybe players or smart monsters might try to move the fight into a location better suited for them.
Narrow bridges, chokepoints, places with lots of pillars and cover, small cramped rooms, large expansive rooms, rooms with rows upon rows of shelves, etc. Furniture, to hide behind, throw, or push up against doors, and so on.
All of the usual things that go around tactical dungeon combat apply in empty rooms, it's just you have to balance how much work you're putting into prepping given that it is less likely to actually get used.
Whatever monsters are nearby or tend to wander around? Clue time! Claw marks in the walls, scorched marks on wood frames, gross-but-informative-to-the-ranger piles of dung, and so on. If it's intelligent monsters, have they left graffiti, signs, or made artistic expressions anywhere? Is everything disturbingly perfectly arranged and absent of dust? (Because, the Lich is very obsessive about keeping things "just right"?)
Again, you have to use care with how much prep time and effort you put in - players tend to skip over these kinds of clues easily, or not connect-the-dots about what they mean. Prep that does not help inform or shape play, even when it's the players being heedless, is wasted prep.
What was this dungeon before? Who built it? Who dwelled here before it was abandoned? Was it abandoned all at once? Was there a betrayal and a civil war between two groups? Was it a magical experiment gone wrong? A disease that wiped everyone out?
This becomes pretty interesting if you can have things like written words carved into walls or ways for the players to decode what happened, and players who like such things. This is the one players usually ignore the most, so you have to be careful about prepping it. Unless you're going to house rule a reason for players to try to puzzle this stuff out ("XP for every relevant history fact you find about this place!"), it becomes things most players don't even think about.
This a careful tightrope, and one fraught with peril. If you insist on making every empty room interesting, you make a lot more work for yourself, both in designing the dungeon up front, and when you run the dungeon. And there is a high risk that you will step over the line (as a few of the other answers have) and make the room no longer qualify as 'empty'.
And then the party may end up spending extra time searching all these interesting rooms. So a dungeon meant to be cleared in one session suddenly spills over to the next.
I would suggest no more than half of the empty rooms should be interesting. The reason they exist is for realism. Most people's guest rooms and second bathroom's aren't that interesting. Glance in and move along. It could be used to hide or prepare an ambush later, but so could one of the previously cleared important rooms.
So for example, a castle might have three identical bunk rooms in the barracks, but only one of them has a deck of cards left strewn across the table (did they leave in a hurry? are their gambling stakes to be found?). Two pantries attached to the kitchen, but only one has an unusual rotting smell (is it just spoiled food? is a zombie waiting to pounce?).
Also remember, what makes a room interesting can vary based on the dungeon. A king's throne in the audience chamber of the abandoned castle is not so interesting. The same king's throne in a random cavern would be (even if it is just there because the orcs hauled it away and use it for seating).
"Empty" rooms don't have to be truly empty, as other answers have elaborated upon.
The furnishings, lack of furnishings, materials, layouts, and other features can all keep the party guessing as to whether anything truly of value or interest is present.
One factor I haven't seen mentioned yet, though, is that the empty rooms provide tremendous flexibility for controlling the pace of your adventure.
If the party is progressing too fast through the traps and enemies, empty rooms filled with more mundane obstacles (stuck doors, piles of rubble that must be cleared before the party can continue, heaps of trash that could contain treasure, worthless trinkets that seem like they could be valuable, riddles,fake maps, misleading clues, etc.) can help you slow the pacing down to something more suited to your plans.
Similarly, empty rooms can be just as useful for speeding up the pace.
Are the players wasting time wandering lost because they can't map to save their lives? A scrap of parchment wedged in a corner might have a rough map drawn on it that leads right to the next planned encounter. A stuck metal door may screech loudly when forced, bringing enemies directly to the party as they rush to investigate the disturbance. Damage to a wall may reveal a corridor beyond that can be reached with a little clearing, thereby allowing the party to take a shortcut around lesser encounters.
Some of these are equally useful for adjusting the adventure's difficulty. If the party is struggling, features of empty rooms can potentially guide them around taxing encounters, or provide useful tools for turning the tide of certain encounters (a wooden table that can serve as a makeshift barricade against rust monsters inhabiting the next room, for example).
Even something as mundane as a ready supply of torches, or a source of clean water, can be quite a boon in the right circumstances.
Likewise, if the party is having too easy of a time, empty rooms can be populated with barriers, obstacles, or environmental hazards that can slowly wear away at the party's resources, or even serve as venues for opponents gathering intelligence on the party's weaknesses, or hit-and-run tactics, ambushes, or plain old deception.
There's some very good answers here. I thought I'd add a few concrete examples to flesh out some of the guidance.
In the classic module G1 Steading of the Hill Giant Chief by Gary Gygax, the dungeon is a working fortress - the base of the Hill Giants. As a result all the rooms have some role to play in its operation. An intelligent set of players will learn things from empty rooms that may help them defeat the giants.
For example, there are empty rooms with beds where the giants sleep. Counting up the number of beds gives you an estimate of the number of giants you might meet and thus the level of remaining threat (if you work through the module, the numbers add up). Even players who don't think of counting may subliminally pick up that there are a lot of giants here so that the whole module will feel "right" to them, even if not consciously so.
A really well designed dungeon will be able to answer questions like "where do the inhabitants get water?", "where do they eat?", "where do they sleep (or do they)?". Even if your players do not ask the question, design like this can make a dungeon feel more "realistic". The fact that you haven't found where the monsters sleep may mean you are missing parts of the dungeon (perhaps behind secret doors you haven't found). The empty rooms may show what is there just by being empty.
G1 (and to a lesser extent the rest of the G modules) are fairly good examples of this approach. It does require more work of course, but in my experience it can create a very good play experience.
Making Empty rooms do something
Whilst this may not specifically be the kind of answer you're looking for, make something they do in one room affect another room. For example, there may be a hidden lever in a torch sconce that when you flip it opens a locked secret door the PC's found in another room they were in before
Giving them a story
Like Magician said before me. Make each room tell a story or give a clue towards some specific goal. If the dungeon is an ancient abandoned room, give a clue to what it's purpose may have been. OR make an important room "empty" until something the players do triggers it. (Slight Spoiler alert) Skyrim's Arkinztham (or however it's spelled) is a good example of this. The final room has a trigger lock that you need to use 2 books you got before to figure out the lock combination. If you get it wrong, several Dwarven war machines come out for you to fight. And the empty pathways let you see how the Earthquake shook it apart, and what kind of technology shaped Dwarven life in Skyrim. Make every room tell a story.
I occasionally run 'Random Dungeons' usually short-term (no more than a dozen sessions), low level game (usually under level 7 or so), using the charts in the back of the AD&D DMG, combined with some of the charts in the 3.5 DMG. The players always have fun trying to figure out what the rooms are for, based on the randomly-rolled 'dungeon dressing'. If there seems to be some sort of emergent theme, I fudge things in that direction, as with one that turned into a yuan-ti temple or base of operation. Now, the players know I'm randomly rolling this stuff, but they do it anyway because it's fun. So, even random stuff (old forge, handful of iron spikes, etc) can lead to some sort of storyline.
Add holes, rifts, or windows going somewhere- you can listen at them, or even look into them... Also just add awareness that something might be wrong.. "The plates of the floor sound hollow or soft".. "The room seems to pulse slightly, in the corner of your eyes."
Very Lovecraftian but always funny - "Inconsistent Perception": "You find a huge wardrobe on the other side of the wall". "I want to look at the wardrobe." "There is no wardrobe here-" "You realize that you are hugging a huge stone obelisk in the shape of a wardrobe, covered with the blood of sacrifices."
Also Cobwebs,hiding everything, blowing in the draught- "You see a huge golem- covered in Cobwebs" "Attack" "You see some Cobwebs blowing in the Wind.."
Also fun- thread the thing to be a deathtrap or one-way. "The orks who build this- seem to have taken no great care to hold back earths pressure- dont touch the rotten beams. Every step seems to make them swing slighty." (Better not cross the room all at once - :)