I used to nod, but then I realized it's too small a gesture to be noticed. Now i wave. And I do it because others do it when I let them in. And most of the time it doesn't waste more than 30 seconds to do it.
Although putting ppl in the rack is a bit much...

You could always turn the sprayers on your car to one side and give them a squirt of windshield fluid (my friends back home used to mess with ppl that way...)...