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.
Remove the shower head and place a chicken bouillon cube in it, then put the head back on.
Put a "Please Use Other Door" sign on the entrance to your office building if it only has one entrance.
Fill someone's hair-dryer with baby powder.
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!
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.
Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
Change the coffee in the office coffee maker to decaf.
Install the Blue Screen of Death screen-saver on someone's computer.