Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
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.
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.
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.
Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
Fill someone's hair-dryer with baby powder.
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.