Worked at a place that got a lot of packages filled with foam peanuts. Saved all of them and put them in plastic garbage bags hid them in every storage closet, underneath desks, etc. Finally had enough to fill up my boss's office. Left the door open a crack, blocked the lower part of crack, filled from the top. Wasn't 100% packed, but sufficient to make a real impression.
I was late the next day, but heard it was awesome. 
Did something similar.
Tilted this guys locker on its back then filled it with the foam peanuts,closed the door and stood it back up and put it back in place.
the look on his face was priceless. and he never left his locker unlocked again.
Different occasion. Using a screwgun. I screwed my bosses lunchbox to a workbench.. We all thought it was funny.
Him.....not so much.
He went stomping into the Dept managers office and I thought my arse was toast.
I was wrong.
The Dept manager also thought it was hillarious.
After thinking about it. My boss eventually did too.
Same boss. different incident.
We were standing in the far end of a long hallway that lead to the parking deck elevator lobby where there were a bunch of visitors were waiting for the elevator.
Now I should mention at this point my boss was a BIG man. Much bigger then me. Easily dwarfed me.
But for all his tough talk. He was like a Baby Huey type, a pure puddycat. (EDIT::question for language MOD. WTF is a thumbcat?)
Anyway.
We are standing there walking about some work that needed to be done in the area when I suddenly got this idea. I looked over and noticed everyone standing at the other end of the hall and nobody looking.
Next thing I did was cowered and hollered out.
"Owe! Im sorry John! Please dont hit me again!"
So now everyone is looking at him and he's stuck just standing there with this big dumbfounded look on his face.
He just looks over at them and says "But,,,I didnt touch him!"
I start laughing, then the people realising I was busting his chops start laughing. Then finally he starts laughing.
Finally he says to me "Im really going to kill you one of these days" LMAO