Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
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.
Put a "Please Use Other Door" sign on the entrance to your office building if it only has one entrance.
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.
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.
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.
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.