Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
April doesn't fool Chuck Norris, Chuck Norris fools April.
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.
Remove the shower head and place a chicken bouillon cube in it, then put the head back on.
Paint a bar of soap completely with clear nail polish so it won't suds up.
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.
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.
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
Install the Blue Screen of Death screen-saver on someone's computer.
Put tape over the optical sensor of someone's mouse.
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!