Pocket Lint has a short article on office pranks, with a few examples.
Such pranks have been a tradition at MS as long as I've been here. One guy on my corridor has a pink office as a result of such attention. Over the years, I've seen people's rooms turned into group lounges, their offices redecorated in a Crocodile Hunter Motif (sorry Joe), including a large talking Steve Irwin, and another office turned into a beach, complete with sand, fake palm tree, lights, and a pool with two fish, one functional, one... well, "less than functional" is the nice description.
Offices turned upside down or on their sides. Computers replaced with Fisher-price models. Pictures replaced. Lots of options there.
My personal favorite is one that looks really bad but isn't. Take a room, and put paper across the window. Fill the space between the wall and the window glass with popcorn. Do the same with the relight (that little window next to the door). Then, build a little cylindrical popcorn holder that hooks to the door, so that popcorn comes out when you open the door. Put something behind the door so it's hard to open. Fill the popcorn holder through the drop ceiling.
Looks like the whole room is full of popcorn.