I've helped my sister pull a few pranks on her office. She works as a lab assistant in a university and shares and office with several other people who are in their early 20s, so you can imagine everyone is pretty well humored and expecting of April Fools Day shenanigans.

The first was terrifying. We cris-crossed the room with a web of fishing line, using more than half of the 100-150 ft roll of wire to make the room near impenetrable. She did leave a hidden path that lead right to her desk, but her co-workers (amid fits of laughter at the ridiculousness) fought through the web the hard way and made sure to turn to their students with expectant looks when they entered the room with questions.

The next year we blew up hundreds of balloons and filled the room knee-deep (We had wanted to fill it up to waist height but it's quite a large room and our fingers were already raw from tying what we had done.)

The last I helped her with was covering people's desks with paper cups (filled with about an inch of water in each) that were stapled together at the lip of the cup in a random pattern so they could not easily be lifted and removed. We provided a turkey baster so the water could safely be removed, albeit over an extended period of time.

There were other pranks and gags which didn't take as much effort...

• Replacing everyone's family photos with "signed" pictures of Lance Bass.
• Suspending a glove from a wire run through the ceiling so it can gently caress someone's head-- being under your control from the other end of the wire on your side of the room, that is.
• Putting tinfoil around someone's entire desk (I suggest if you're going to do this also add a layer of clear cling-wrap so the removal is even more ridiculous).
• Filling desk drawers with packing peanuts (we used shredded paper... recycling at it's best!).
• Turning furniture and objects in the room upside down, or turning them 90 degrees from their original position.
• Changing the desktop background of everyone's computers to a movie file of Rick Astley's "Never Gonna Give You Up!"