Ceramic Jar of Acid (CR 1)
The door into the room is slightly ajar.
Type: mechanical; Perception DC 18; Disable Device 16
Trigger touch; Reset None
A ceramic jar of acid is balanced on top of the door. If opened, it falls on the character. Jar of Acid(1d6 damage plus 1d4 acid damage, DC 18 Reflex Save for 1d2 acid damage)
This one is so obvious that whichever PC gets hit by it just may earn a new nickname. They'll all groan and won't believe that they got hit by such an obvious trap. Of course, some time later, you should do it again, preferably after they disarm the trapped floor in front of the door, or are invited in by the room's occupant. If the same person is hit again, he'll definitely have a nickname.