One of my favorite things to do when I was younger *cough cough* was to wave to people in passing cars (when I was a passenger, of course, not while driving) and see who waved back.
My kids are positively wicked about doing this. On road trips the various "games" to be played with other motorists include smiling and waving but then holding up a handwritten sign trashing whichever political candidate the other car had on its bumper stickers. The immediate contrast between cheerful smiling and waving back to angry scowls was rather interesting to watch. Another favorite of my son's was to appear to be picking his nose (the bent knuckle method looking like he was really digging for gold) and then sticking it into his mouth just as we were passing another vehicle, making sure the driver or passengers had seen him. Sometimes he would put on a great theatrical performance of appearing to carefully scrutinize his finger tip before inserting in mouth. That was sure to get howls of laughter from his sisters and much discussion as to how this was received by the people in the passing cars.