Eyeing for the perfect trap, puzzle or tricks to keep your friends eventful.
A room filled with sharpened bamboo poles sticking out of the room at evenly spaced intervals, floor to ceiling. There's not really enough room to just slip by, and if one of the poles is touched, it comes alive, dealing 1d4 hp damage to the character.
A long, windy passage with grates spaced at even intervals along the floor. A closer look will reveal unnaturally smooth walls, the surface almost glassy. Before they get in too far, the door they came through slams shut, opening a wide pipe that starts filling the room with some liquid as all the floor grates seal. The liquid could be simple water, or, if you're feeling particularly nasty, make it something more dangerous, like sulfuric acid. You can also make the door at the other end of the tunnel a puzzle to open too, for added fun and panicking.
The players find an important door they have to go through, but a massive pillar has fallen directly into it, jamming it and blocking the way.