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.
Change the coffee in the office coffee maker to decaf.
Hide an alarm clock in someone's bedroom and set it for 3:00 a.m.
April doesn't fool Chuck Norris, Chuck Norris fools April.
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!
I just recently discovered that there is a national holiday named after Atheism. April FOOLS day. Like this story in the name of Jesus.