Remove the shower head and place a chicken bouillon cube in it, then put the head back on.
Fill someone's hair-dryer with baby powder.
Write a message on an upside-down paper cup that alludes to something horrible being trapped under it. Leave it on a coworker's desk or in a conference room.
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
Put a "Please Use Other Door" sign on the entrance to your office building if it only has one entrance.
Pull on a coworker is to press ctrl+print screen on their workstation, then paste it into Paint, save the pic, and set it as the desktop background. Move all of their icons to the trash. When they get back to their desk, clicking won't accomplish anything!
Install the Blue Screen of Death screen-saver on someone's computer.
Take a squirt gun into the rest room stall next to someone and shoot little drops over the wall every couple of seconds while pretending to pee.
Put tape over the optical sensor of someone's mouse.
Hide an alarm clock in someone's bedroom and set it for 3:00 a.m.
Paint a bar of soap completely with clear nail polish so it won't suds up.