Trollsmyth has some cool ideas about applications for the timed dungeon doors thing. This is one of the things I love about blogging. Bouncing ideas off of people, and ending up with things that are even crazier than what I come up with. And, of course, it gives me a good clue to what ideas are actually interesting.
The whole thing got me thinking about puzzles. The calendar/timed doors make pretty good RPG puzzles, as long as the GM is willing to keep track of them, and they don't end up making bottlenecks in the dungeon and stopping the session while one person solves them.
If I worked up a labelling system, and turn them into a cipher-type puzzle that gets easier to solve the more doors they solve, the puzzle people would be really happy. But I've also got a player who just likes to take notes, and keep track of things; even after the puzzle had been solved, she'd still get a kick out of being able to tell the rest of the group which door to go back to.
The best part about these sorts of puzzles is that they're fairly well integrated into the game part of the activity. A puzzle based off of the calendar the DM uses, that you can only solve by exploring the dungeon, and rewards you with even more dungeon, isn't the kind of thing you can pick up in a drug store. And of course, building a bit of world info into a puzzle, a challenge the players can solve, gives them a reason to actually pay attention to the background for once.
Even better, though, are puzzles that I think of as "environment puzzles." I give the players some not-immediately-obvious situation, say, some bit of treasure on a pillar in the middle of an underwater lake. I give them a sketch description of the area, maybe make some modifications if they ask a question that suggests a particularly interesting possibility, and they figure out some way to get to the treasure. The fun part, of course, comes with rolling the dice, because then their plans never go quite the way they want.
I figure most people are already doing this. I got a lot of it, originally, from Roleplaying Tips #5: How to Turn Brain Teasers Into Amazing Roleplaying Opportunities. But this is the kind of thing I didn't know, when I first started out, and I'm still trying to work out, after having been at it for a bit: how to play to the medium's strengths.