Slapping cars running pedestrian lights or zebra crossings (takes skill as you can lose your hand), and similar for artic vehicles including my favourite bendy buses for stopping across crossings. Slapping is an art as you clearly don't want to do any damage, nor make it is clear what you did as this risks road rage.

Chucking lit fag buts or other litter back into cars after they have chucked it out of their window. A little more provocative.

What a sad little life I lead.