Car prank ideas that won't cause any damage

Just seen the black pecker tire valve cap thread and reminded me I need to start thinking of a good prank for my upcoming trip with some friends.

Every year we go to Moab and there's always Shenanigans.

Last year we all got the black pecker caps.

I want to return the favor somehow.

A few ideas so far.

Placing a Harmonica and placing it in his grill.

Thick heavy zip tie around the drive shaft

A super thick balloon filled with rocks and put over his muffler to sound like something is broken. (Works really well)

Any other ideas?

These will be going on Jeeps