Keying a car is number one. That's where you drag your key over your cheating boyfriends car. Ruin the paint. Scratch your name in it. PS: you can get arrested for this. Other brilliant ideas other than move on to the next idiot....slash tires-smash windows-leave dirty messages in the car dirt- firebomb- sugar in gas tank- push car off tall building...I made up the last three. sorry.
Here's a tip ..If he cheats it's cause he doesn't see your true beauty- someone else is just waiting to appreciate you.