Not that you should but is always fun to...

-Pour brake fluid on their car so it eats off their paint. The average person #1 has no idea that brakes have fluid and #2 have no idea that it would eat paint anyway.

-Remove their valve cores from the valve stems. This is great because it is impossible to hold air in the tire yet you didn't damage the tire. This is great because #1 the average person doesn't know what a valve stem is and #2 don't have the special tool needed to replace them.

You work on cars for a while and you learn a thousand different ways you could destroy them.