Here's a couple techniques I used to employ when having fun with a fellow coworker. One requires X being the windowing client, but the others are sufficiently "entertaining" IMHO :-)

• (requires X and access control enabled if ran remotely) Make a perl script to use xset(1) for alternating the mouse acceleration from 1/1000 (incredibly slow to go across the screen) to 10000 (move a quarter inch and it's gone!). Have the script fire off randomly and sleep for up to 3 or 4 minutes then reset mouse speed to "normal". Having the script swap buttons (or make all the "left" one) as well is always a nice touch.
• A perl script that'll randomly choose a printable character and use stty to remap that key to the line kill for the youth's terminal(s).
• Make replacement versions of common commands (I prefer 'ls') that are a bit sassy. For example, every N times ls is ran, have it refuse by saying something like "nah, use your memory!". Bonus points if he does it by command aliasing and then grabs the type, which, and whence commands to make the new versions tougher to track down (obviously he'll need to stealth in the aliasing).

Enjoy! :-)