I've had a couple 'Summers' that I really had to give a hard time to, some of my favorites:
Attached a wireless mouse to computer: One of our students sat by the coffee machine, so I attached a wireless mouse to his computer, then everytime I went to get a coffee I'd move the mouse around and listen to him scream at it. Later, I conditioned him by stopping everytime he slammed the mouse down hard on the desk. The apex was when I let the MD in on the joke. So the MD goes in to ask him some questions and show some numbers, on cue I start moving the mouse. The MD starts chewing out the kid "God damn summer students can't even work excel...". So beautiful.
Turned one screen upside down and the other 90 degrees to the right and set the backgrounds on his computer to look right side up.
Wrote two macros as CTRL+X and CTRL+Z. CTRL+X was remapped to conditional format all cells and contents to white, which meant that while the data was there, unless you removed the conditional formating, you couldn't change it back to black. Of course, as soon as you screw something up, you hit CTRL+Z, which I mapped to change all column widths to 99999 and row heights to 99999. Also no damage done, but scared the crap out of this kid.
TLDR: Don't leave your computer unlocked if you fuck up and cause a lot of work for the Associates.