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.
Remove the shower head and place a chicken bouillon cube in it, then put the head back on.
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.
Put tape over the optical sensor of someone's mouse.
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.
Install the Blue Screen of Death screen-saver on someone's computer.
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.
Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
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