I always had a cool door idea.
a 10 by 10 room has 162 doorways in it, each one five feet wide and lining up perfectly with a battlemat's gridlines. Each square has 4 doorways (except for walls and corners), created by 4 posts at the 4 corners.
Now, there arn't as many doors as doorways. Say, only 60. But they are magical doors, and lack hinges. But they are also magically powered. Just a touch will set them swinging exactly 90 degrees in the direction of choice (logically, pushing on the left side sets it swinging right, and vice versa (as with any real door, the hinges are on the opposite side of the knob).
Doors can only swing away from the touch.
In any case, once the door swings 90 degrees, it becomes a barrier between two completly different spaces, and therefore can again swing either direction. So, for example, a door could be swung left, right, left right, to move it diagonally. That is to say, the doors dont actually have hinges, and so can move around (they arn't tied to a single post).
now, this by itself is interesting, but what makes it cooler is when doors hit each other.
If a door is opened left, but there is a door in that frame already, that door gets "bumped" into its next doorframe, opening the same way as the first. So, for example, if a post had four doors, pushing one the corect way would cause all of them to swing 90 degrees, pushing you into the next space (and pushing all the other people nearby in the same sort of way).
Now, this room is a bit boring if every doorframe is filled. But if a whole bunch are empty, it becomes a room with what are essentially movable walls (which move in predictable ways as explained above).
Alone, this makes a pretty good puzzle room, especially if one door is unmovable, making it impossible to budge doors that would want to take its frame.
But if you put a combat in it, it is a really easy way to split the party, and keep them in the dark as to where the enemies are. Combine this with soundproof doors, and suddenly everyone is incognito. Party memebers could accidentlly work agaisnt each other, pushing each other back as they move forward. and they get attacked to boot.
Add some pit traps, and your friends can accidentally shove you into thin air and a long painful fall.
The possiblities are endless.
Although I don't think i explained this very well. If you want i'll draw up some pics and post them to explain better. Its a bit tricky to visualize.