The good old "x-acto knife leaning against the back of someone's phone"... that's always good for a laugh.
But seriously. I used to share some cube space with an electrical engineer. A good portion of his job involved soldering together prototype circuits. One time when I knew he was working on designing and making the prototype of a 220V power supply, just as he touched the soldering iron to the circuit board, I twisted a roll of bubble wrap right behind him.
You should have seen him jump.
Another time...
He was kind of turned to his left, reading some schematics. The soldering iron (still on) and the other tools were over to his right, now basically behind him. It was a Friday around 4:45, and I was talking to some of the other guys, telling them how I just finished one thing and didn't want to start another in the last fifteen minutes before a weekend. Then I said "I know I'll just pick up the soldering iron and poke Rob with it." I know he heard me pull the iron out of the stand. But just as I was saying, "...and poke Rob..." I poked him with a screwdriver instead. You should have seen him jump.
My one boss has a snow globe where you can put your own pictures, sliding them in through the bottom. She normally kept a picture of her husband and newborn son in it. I took a picture of myself, printed it out and snuck it into the snow globe when she was at lunch. A couple days later we got together to discuss allocating people to the projects we had coming up. When I got to her office, my picture was still there. Part way through the meeting, I leaned over her desk to point where a new task could be inserted inthe Gantt chart and "just happen to" notice the picture. I said, "Aww that's sweet and all, but does Jeff (her husband) know about this?" She hit me.
My current boss usually works 8 - 4:30 or so, while I'm usually an hour or so behind that. Last December, when the mail room guy came by to deliver the refill for his dayplanner, my boss was gone and his door was locked, so the mail guy just left it leaning against his door. I grabbed it and sliced the bottom of the plastic open. I then slid out a page and wrote "Give Ryan a raise" on one of the Monday pages, and put the page back in. I had completely forgotten about it until we got to our Monday Morning Meeting on Jan 30 and he started laughing. I didn't get the raise.