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.
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.
April doesn't fool Chuck Norris, Chuck Norris fools April.
Paint a bar of soap completely with clear nail polish so it won't suds up.
Hide an alarm clock in someone's bedroom and set it for 3:00 a.m.
Put a "Please Use Other Door" sign on the entrance to your office building if it only has one entrance.
Install the Blue Screen of Death screen-saver on someone's computer.
Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
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.
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
Change the coffee in the office coffee maker to decaf.