If they are stopped at a light, I think using a small awl or something to subtly perforate, say, their right rear tire would be cool. You could "key" the paint as you ride by using just about anything. The possibilities are really endless here, you just have to be creative. My problem is that whenever I encounter someone that I feel really deserves it (and I'm mostly pretty passive so this takes a lot), I'm not carrying anything useful on me. It's kind of interesting - on my ride to school I ride through some not-so-good areas for a few miles and keep my u-lock clipped to the outside of my bag, and have had nothing but good experiences with drivers. On my 5-block ride to work down a somewhat busy street in a nice neighborhood not carrying anything but a paperback, I swear people are trying to f-ing kill me.