Whenever I'm writing an adventure or dungeon, I have these vague and shifty ideas about what has to be in there and I realize after a while that it doesn't feel right unless I've filled in all of the "slots".
This isn't a list of everything that should be in an adventure, it's just an attempt to pin down the things I usually find myself feeling like I need to get in there last thing at night before I go "Ok, I can sleep now."
Rabbit hole: Something the PCs can do/discover/get interested in that will completely change what the whole session is about and force me to improvise everything.
Tactically complicated set-piece fight: An area to fight in where something fucks with normal combat considerations--teleporter, random moving magma pit, etc.
Some completely fucked weird element I am inordinately proud of. Note: PCs may not even notice how fucked it is--like the chick whose whole body was full of tiny spiders who the driders use as a living trap and whose head explodes when you come near her, covering you in spiders.
Some almost-as-fucked weird element which is designed specifically so the PCs can interact with it and therefore make the whole situation more weird in their own way if they choose to: Like a vial of water that makes a reflective puddle and that puddle is actually a portal to a mirror-universe full of exact duplicates of whoever's in this universe who want to kill their analogues a la a clone spell.
A new monster. I try to never use the same monster twice if I can unless there's some setting-reason it should be there, but even if I do, I always want some new beastie in there. At home, it's usually one I made up--less so on TV since it's fun (and novel) to get to use the miniatures Reaper sends and when the players go "What is it?" I get to go "It looks exactly like that."
Weird details to attach to every stock monster used. Manticores like poetry. Did you know that? They do. They will obey the best poet on the battlefield. On a 1 or a 2 they're ok with poetry that doesn't rhyme.
Outside-of-standard-game-rules 4th-wall breaking gimmick--i.e. a 3d visual aid that you can do something to or a concrete real-world puzzle to solve or cued-up sound effect or bizarre piece of terrain made from some obviously-not-made-as-DnD-terrain thing like a taxidermied animal.
Something screwy the bad guy can do to you besides just hurting you that makes the game weirder from then on.