I've been playing since about `79' or so, and one of the most devious traps that I've ever seen consisted of a long tunnel that was a few feet below the level of the passageway that the players are currently using, IE the players have to descend a few steps to enter this part of the tunnel. Inside the tunnel the air is mainly carbon dioxide causing torches and players to flicker and die. The trap comes in the form of the vaulted roof of the tunnel containing not Co2 but methane. If the players think that by raising their torches they will get to the better air above the carbon dioxide then they get a big boom, if not then they pass out and die in the dark. The beauty of something like this is that it is not 'officially' a trap.

You could also have a wizard in disguise selling frozen meat. This is good because the second night out the adventurers can have fresh meat for chow... The problem is that the meat is in reality, troll meat. And will regenerate into pissed off troll(s) while the adventurers are sleeping.

BTW...My current character is Sir Justin Thyme, Knight Protector of Chappalton Island, 13th level human thief, Thieves guild founder and guildmaster of the Chappalton Island Thieves Guild, Spymaster and advisor to Baron Warren, and Lady Lantha the high priestess, Master jeweller, Innkeeper and Patron of the arts.