Blueberries!
A handfull of berries, or chokeberries anything that causes them to take notice, and won't get you arrested for damage or attempted murder.
They sound like BBs.
And you can blame that flock seagulls overhead. 

You gonna ride around with a pocket full of blueberries? Have you seen the price of those things? 

I'll eat em before I throw em. 

Mmmmmmmm, blueberries.

Ping pong balls... Cheap, looks like a golf ball, bounces in the most erratic manner nd does absolutely no damage... You don't even have to throw them over your shoulder. Just drop it and let the air currents do the rest. 

My licence plate reads "ARMED". I don't get a lot of tailgaters.
There's also this option:

My licence plate reads "ARMED". I don't get a lot of tailgaters.
There's also this option:

Where did you get that shirt? I soooo need to get one of those. That's awesome.