There are a number of applicable classics.
The cat turd reminds me of an old one involving sardines and a heater. That only works if you don't share his airspace.
One of my favorites is the old key copying trick. Obtain a copy of his car key. Then make a daily habit of changing something small on his car. Change his radio station. Leave his lights on. Leave it in a different gear than he left it in. Move the seats. Gradually move up to more and more drastic changes: turn the car around, or park it someplace different -- not a lot different, just different enough to make him think he's losing his mind. That's a long slow prank, but it's well worth it in the end.
For office filling, newspaper also works well. It takes a while to wad up enough newspaper to really be effective, though.
If he has a tile floor, it can be a lot of fun to wax inappropriate material into the floor. That may be over the top, though.
If his office is small enough, you may be able to move the entire setup into the nearest bathroom. Or out into the hallway. Or onto the roof of the building. The real trick is to ensure that it's exactly like it was -- take a few digital pics to make sure you've got it right, and this one is well worth the time that it takes to get it right.
If his office is big enough, transplant a porta-potty into it. Be careful, it can be against the law in some places.
Do the car trick, except with his office. Wait until he leaves, then go in and move something. Like the car trick, just move small things at first. Shift his whole office around just slightly. Change his chair height, or move his keyboard, or put his mouse on the other side. Just enough to make him doubt his own memory. Takes a while, but it can really be worth it.
Let us know what you end up doing -- I always love new ideas.
