Awhile back, I turned a cup of BB's upside down on somebody's desk. The first cuss word was heard before the BB's even hit the ground, and we were all laughing for twenty minutes afterwards.
We are still finding BB's all over the place, especially when the vacuum makes its rounds.
Step 1: BB Spillage
Put something firm on top so you can flip it upside down without spilling, such as a coaster or piece of card stock.
Turn it upside down on desk and slide card stock out.
Put it somewhere to the side of work area so as not to raise too much suspicion but yet annoying.
Use the plastic BB's if you don't want the person to clean them up easily with a magnet. The metal ones make a great sound tho.
Cut a hole in the bottom of a large cup and fill it half way with BB's. Put candy or cheetoes or something in the top to cover up the BB's.
Placed on top of a door.
Taped to a cubicle storage door so it slides and spills when the door is open.
Put a whole bunch of cups all over the cube and fill some (or all) with BB's.
Use your imagination.