Making Pay Back a B*t*h

OK, I need ideas of stuff I can do to my friends cars that WON'T fuck them up permenantly

Here is the deal, all of last summer we would pull pranks on people from "stealing" their car, to toilet papering, to seran wrapping, SO I need some good ideas to get them back. Last night we were all hang out and they stole my atennea while we were stopped at a red light, so a little later I got chalk marker things and wrote on their windows, so as my pay back for that, they went to Wal-Mart, while I sat in the parking lot waiting for them, when they came out they first threw water balloons at my car, then silly stringed my front window, then, this is the big one, threw paint grenades at my car, so My car was wet, had hot pink and green silly string on the front window and grey paint all over it(oh the paint was water based so it wash off). But as you can see I need a good payback, Sorry for the book, but HELP me out.