Paint a bar of soap completely with clear nail polish so it won't suds up.
Fill someone's hair-dryer with baby powder.
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
April doesn't fool Chuck Norris, Chuck Norris fools April.
Change the coffee in the office coffee maker to decaf.
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!
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.
Install the Blue Screen of Death screen-saver on someone's computer.
Remove the shower head and place a chicken bouillon cube in it, then put the head back on.
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.