Paint a bar of soap completely with clear nail polish so it won't suds up.
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.
Put tape over the optical sensor of someone's mouse.
Fill someone's hair-dryer with baby powder.
Install the Blue Screen of Death screen-saver on someone's computer.
April doesn't fool Chuck Norris, Chuck Norris fools April.
Remove the shower head and place a chicken bouillon cube in it, then put the head back on.
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!
Change the coffee in the office coffee maker to decaf.
Find a sleeping person, fill their hand with shaving cream and then tickle their nose.
I just recently discovered that there is a national holiday named after Atheism. April FOOLS day. Like this story in the name of Jesus.