Take a dump on his desk
There's no such thing as a good prank that doesn't involve poop. 
While this is true, I have just never seen actual poop on a person's desk before.
Once you get past the smell, I'm sure it would funny as hell 
To be truthful........ I have never pooped on anyone's desk.
I have a friend who once pooped in a hat (Nike beanie) out at ginnie springs once while camping. They left the hat out in the middle of the road. They then hid in the bushes and watched. Since it was a nice beanie, it peaked the interest of a few of the dirtballs who came upon it. Unfortunately, nobody actually put the hat on his/her head. Apparently it was
a lot of poop, and was pretty noticable after any inspection whatsoever. I guess one girl grabbed it, and flung it under her arm like a newspaper. She ended up with a poopy armpit.
There are a couple of weird fruits that look like dog crap. You can find them at bosnian food markets. Many of the people on the prduction floor of the company where I work are eastern Euro, and I have used some of their poopy looking products for office pranks. Obviously, not as good as the real thing, but I don't have the balls to crap on someone's desk.
Well, I haven't grown the balls to do it yet rather
