Tennis balls, and especially golf balls are my favorite. I shoot my blunt-tipped rabbit arrows at them. When you hit a golf ball good and square with a blunt, boy does it go flying! Lots of fun, very good reinforcement.
I used to shoot thistle tops with judos, and those blow up nicely like dandelions, but you've got to really nail them. The problem is that then I was spreading thistles, so I figured I'd better stop. :(