Out of all the dozens of humanoids in D&D, I think my favorite has to be the kobold, particularly how they were portrayed in 3.5. They were small, dextrous, and deviously clever. Kobold lairs were like miniature versions of dragon's lairs: lots of traps, lots of areas where the characters were vulnerable, and devilishly tough to survive. Any trap you could possibly think of, the kobolds had at least one.
Their lairs were small, cramped, tough for human-sized creatures to move in, and absolutely lousy with traps. Rolling boulder traps, spiked pits, acid-spraying mechanisms, completely empty chests protected with poison needles and spiked walls that closed on the characters; you name it, the kobolds had it, and the worst part was that the kobolds were always in a position to counter attack the characters at their most vulnerable.
Not only were the kobolds masterminds of traps, but they were the descendants of dragons. There was always a good chance to find some kind of draconic item or a dragon-blooded kobold at one point or another. Even at higher levels, kobolds were still a threat because of the sheer deadliness of their lairs and the love of wealth they shared with their draconic ancestors.
Possible Traps in a Kobold Lair
1. Empty chest with a pressure plate in front of it attached to a poison dart mechanism.
2. A deadfall – if characters move a specific piece of treasure or hit a pressure plate, the ceiling collapses.
3. A pit with a gelatinous cube at the bottom and greased walls.
4. Two logs hung from walls that smash together when the trap is triggered, crushing anyone between them.
5. Vials of alchemist's fire that drop from the ceiling onto the characters when the trap is triggered.
6. A floor that sinks into a pit of acid when too much weight is put onto it.
7. A trap that sprays the adventurers with oil, in a room filled with candles and a fire pit.
8. Hidden blades set into the wall, high enough that the kobolds are in no danger, but that can swing out and decapitate larger adventurers.
9. Spikes that spring out of the floor, stabbing into adventurers' feet to hobble them.
10. A round boulder of the perfect size to roll down the sloping, conveniently round tunnel.