I have accumulated a lot of D&D 3.5 modules and adventure settings, but I play older editions of the game. What is the best way to convert these to an older edition, say AD&D 2nd edition? Is there a "quick fix" that makes conversion easier, or must it all be done one "piece" at a time?
I don't think there's a quick fix, but I do think it's something you can do on the fly once you've got some practice. The idea is to used the 3.5 material as a guide for what to put into the game rather than as a (more, or less) strict recipe for the adventure.
For fights, it depends on how you run your AD&D. If you don't worry about encounter balance and prefer to leave it up to the players whether to fight, run, or invent a third option, then you can run modules as-is just by dropping in 2nd edition creature stats for the appropriate enemies. (If you do care to balance things as much as 3.5 assumes, then I've got nothing. Sorry!) Essentially, read a set of detailed stat blocks for six 2nd level Orc Warriors and one 5th level Orc Warrior chieftain as "six orcs with good gear and maybe +2hp and one orc with great gear and full hp."
The balance won't always be the same—some fights will be easier and some harder—but since 2nd edition isn't so tactically-oriented, a well-matched fight isn't usually the play objective. For creatures that don't have 2nd edition equivalents, either reskin an existing creature or just create a new creature inspired by the 3.5 version. The idea is to take the 3.5 material as the guide to what 2nd edition material to populate the adventure with.
You can do pretty much the same with magic items. Some will translate 1-for-1 between the systems. Others will be weaker or more powerful, which I personally think is fine. Some things, like tindertwigs or whatever "mundane" magic items 3.5 had, and items that are specific to how 3.5 does things, you might not want to keep—in that case you can go straight to the good ol' random magic item charts and have fun replacing them with 2nd ed stuff.
The harder part is going to be where the 3.5 module assumes a robust skill system. For physical skill checks you can fall back on ability checks with or without modifiers, or just let them do it if they've got a well-described plan for how to get the job done.
For social skill checks you can use the skill suggestions as a guide for how to roleplay the NPCs. If the 3.5 material is using a (say) diplomacy check as a fork in the road for how the module will go, replace that by asking yourself a question: "What could the PCs do for/give to the NPC to get what they want?" Once you know that, let it come out when the PCs interact with the NPC. Don't make it hard to find out or necessarily even hard to do—concentrate on making it something that will make the PCs really think about whether they want to grant that concession to the NPC, or whether they'd really rather not change the NPC's mind. And, of course, allow for creative alternatives that the players might concoct to influence the NPC…
Not all situations are going to have that obvious kind of plot structure, though. In 2nd edition, when I really don't know how the NPCs are going to feel about the PCs and I just want a roll to tell me, I often use the NPC reaction chart to find out what their disposition is, and then mentally keep track of how that might change up or down as the PCs try to influence the NPC. There's no solid, systematic way of doing it, but I find it works with practice and a generous GM spirit.
All of this presumes that you're playing closer to a 2nd edition feel. If you really want to be faithful to the 3.5 material, then more work is going to be required. However, most of the time you can probably get away with converting just the core bits of the 3.5 material one piece at a time before play, and using these techniques to smooth out the less-critical sections in between.
I upvoted SevenSidedDie’s answer, but here’s my take, for what it’s worth...
No matter what game system the adventure was written for or that you’re using, this is how you convert on-the-fly: Simply ignore any stats or mechanics in the module. Pick an appropriate monster, treasure, or mechanic from the game you’re running based only on the text descriptions in the adventure.
When there isn’t an exact match, pick something similar. Then you can either simply make it the thing you picked instead or use the stats/mechanics for what you picked and describe it to the players as what the adventure described.
For the d20 System to older D&D’s specifically, though: For most skill checks, first ask yourself if it works for the PCs to simply succeed automatically. The d20 System adventures have a tendency to call for a skill check where before none was needed.
Swapping out "normal" monster stats is the easy part, just replace the 3.5e manticore or whatnot with the 2e one. The NPCs and levelled monsters are harder. Something like a goblin or orc, you can just treat as a levelled human, but weirdo stuff may present more of a challenge. Just swap it out for something else.
Keep in mind difficulty levels go up from 2->3->3.5->Pathfinder - assumption of party average hit points and DPS goes up sharply, so even with swapping in monsters do NOT try to run a 3.5e for "levels 5-7" for a 2e party of levels 5-7. It'll be like twice as deadly.
Really you can do without skill checks, because the 2e and earlier mode of playing didn't rely on them - we used to just let the PCs do it, or make a stat check, or let them figure it out with clues and player knowledge. Besides, most 3.5e adventures don't formalize skill stuff like they're 4e skill challenges, they just require Search DCs to find stuff or Balance DCs to not fall down. Replace stuff with thief skills where appropriate and stat checks where appropriate.
[Although - one of my biggest 2e house rules was a Perception stat. Then when 3e came along and rampant Perception/Search was there, it wasn't much of a change for me.]
Just replace the monsters with their 2e stats and see what happens. Give it a go. If it's looking too easy or too hard then adjust it on the fly.
Even if you are rolling your monster roles out in the open you can still fiddle monster hit points and tactics whilst the battle is going on.