I had the characters go through a mountain pass that was essentually a 10ft ledge with the mountain going straight up on the left side for 50ft and on the right side it was a steep 80ft drop (Essentualy a pass created a long time ago by an ancient culture) The pass eventualy led to a cave system recently inhabited by a few hundred Kobolds who fled slavery a few months back; so therefor the ledge had traps. I Deseigned my own little bear-traps to use along with a neat fence that blocked the way (cutting the middle wire or messing with the posts would cause a thorny net to fly down from a camoflauged rest on the cliff face.)
But, the most interesting trap I through at my players was one useing the Kobolds themselves. The situation was they carved four 3ft around holes in the left cliff face 40ft up, they would then crawl on their bellies and toss alchemist fire and large stones (for comedic effect and show that the Kobolds weren't as smart as they so-far apeared I had a few acidently fall to their deaths trying to throw barrels full of alcahemist fire at the PCs). Anyway, it was designed in such away that the only solution was to run past as fast as possible (into an ambush of course.)
Now, my players really enjoyed that type of "liveing trap" and asked if I could throw another in at some time.
My problem is...I can't really think of one. I keep trying to pick out different enviroments I could exsplot like I did for the first one, but I simply can't think of anything else. Anyone out there with any idea's concerning an area the enemy is useing that leaves only creative or "flee" type options for the PCs? (Certain spells and items could have helped with the Kobold Cave thing; but the characters didn't choose any of them-- the players had fun discussing them later on.)