What about the good, old `Death comes crawling through the pipes`theme?
Basic Layout would be a spheric room, maybe even a cube. The group enters from the south, only to find themselves trapped by a (magically hidden) portculis blocking their way back (or any similar trap door) to the area they previously left.
Roll initiative. A succesful Arcana/Thievery check(easy DC) will tell them it will take lots of damage to force it open again and a succesful Perception check (hard DC) will tell them there is more to the strange pipes in the northern end of the room than meets the eye (signs for the presence of a levelappropriate monster). And just when they were sure things could not possibly get any better they find out that the room has more traps than Halaster`s Undermountain. Some great traps include crackling floor parts or pressure plates, that, when triggered, release venemous gas into the room (for serious ongoing damage, so the players have an incentive to try to leave the room).
The group is now confronted with three choices:
-Try to break open the portculis before going down due to the poison damage and without alarming the lurking monsters
-Try out one of the pipes (Athletics check to reach them) and bring the fight to the baddies
-Make a last(?) stand and lure the monsters ourt of the pipes, so that the poisenous fog might affect them as well and so they do not have to fight the monsters in such an incovenient place as a pipe
Options 1 and 3 are pretty straightforward to design, while Option 2 might be a little trickier to design.
I`d suggest you have the characters in the pipes grant CA for fighting in such a confined environment and have them march like ducklings so you have the opportunity to hack at them anyway your monsters like to. Build for a narrow escape, let them see the light of day and...what follows should depend om how much more your players can take
If you really like to put some work into this room you could of course build three different pipes where each of them has her own inconviniences to find out about but most groups will only see one of them anyway.
