Rig a party popper to detonate when your victim opens their desk drawer. Use strong tape to attach the party popper to the inside of the desk, right above the top drawer. Close the drawer almost completely, then tape the string of the popper to the inside front of the drawer (if necessary lengthen the string slightly with another piece of string). Be sure to angle the popper so it will not explode in your victim's face.
Submitted by Office Troll Oloz1
This one takes a small investment, but it's worth it. Borrow one of your victim's business cards and have it duplicated, but change one small thing--the spelling of their last name, or their job title, for instance. Sneak in and replace your victim's cards with the new ones. When the time is right, casually 'notice' the mistake. Your victim will freak out thinking they've been handing out business cards with a typo on them! (Can also be done with their resume, important projects that they've already turned in, etc.)
Submitted by SuperDude32