As long as a prank can be undone it's likely harmless. Covering a yard in AOL discs (remember those?) does almost no damage and creates a cool effect on a dewy morning. The same goes with Saran-wrapping a car or filling an office with packing peanuts — as long as you're willing to help them clean up. Where pranks cross the line into malicious is when something or someone is irreparably damaged. Maybe they had it coming, but it's something to consider when contemplating a prank. We're not saying this SUV-driving asshat deserves having his car destroyed, but we support maximum_sarge in his quest for vehicular mischief.
My favorite act of SUV retribution is to swap the little rear wiper blade for the driver's wiper blade.
Manually folding or adjusting their mirrors is also fair game.
No damage except to the psyche.
If they've really been a douche, then crossing the pin-out's on the trailer hitch with a paperclip is the next step.
Both new on us and worth trying.