Fill someone's hair-dryer with baby powder.
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.
Place a pair of pants and shoes inside the only toilet stall in a rest room to make it appear someone is using it all day.
Put a "Please Use Other Door" sign on the entrance to your office building if it only has one entrance.
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
Paint a bar of soap completely with clear nail polish so it won't suds up.
Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
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 an alarm clock in someone's bedroom and set it for 3:00 a.m.
April doesn't fool Chuck Norris, Chuck Norris fools April.
Install the Blue Screen of Death screen-saver on someone's computer.