best revenge/payback...

I've got a couple friends I need to payback for some things they've done for me. I don't like messing up people's cars and they are my friends, so I dont wanna do anything too bad. So what have you done to payback somebody? What are your favorite revenge tactics?
 
I'm all for violence personally, but that's me. The answer really depends on how nasty you want to get? I mean you can go from egging and rolling their house all the way up to embarrassing them rather severely with their significant others.