What are some general tips for increasing the challenge as the campaign goes on and my PC's out level the suggested challenge rating.
So there are 3 mains types of encounters in the game. How you level these will vary.
Type 1: Dealing with NPCs
Most of these are "skill" interactions that involve doing things like rolling
Diplomacy checks or fasting-talking NPCs into working with you. If you want to "scale this up", you just give the NPCs higher checks on things like
Type 2: Problem solving
This involves things like "hey we need to cross that rickety bridge" or "how can we sneak into the room without the guards seeing?".
The key balancing point for these types of problems is generally the access to magic. Hiding from guards is easy with
Invisibility, crossing rickety bridges is easy with
These are always really difficult to "scale", because you are one DM and they are 4+ players. Regardless of level the players will often find ways to surprise you. The best you can do with these is just to make them fun and not "too easy" given the magic / skills the PCs have available.
Type 3: Combat
The simplest strategy is just "extension".
- Simple enemies are generally pretty easy to upgrade. Honestly, if you want to upgrade a "boss" quickly, just add 2 to everything. Give them +2 HD, +2 AC, +2 to hit, +2 on DCs etc.
- The other common tactic is to add more "minions" to a fight. Fighting a boss and 10 orcs is easier than a boss + 15 orcs.
- You can also extend a day by having more combats within a day. They don't need to "lethal", they simply need to be enough to cause the PCs to spend a couple of spells / resources.
If the players are 2+ levels ahead of "expected", you may need to resort to "substitution". In some cases, this is easy as you replace "goblins" with "hobgoblins" or "small elemental" with "medium elemental". However, some creatures that are appropriate for 3rd level characters are simply inappropriate for 5th level characters.
This runs the risk of needing to change the story, so try the "extensions" method first.