Blow up several balloons and attach them to a cheap wooden frame of some sort. You can even use multiple colors and "call your shot". You'll know if you hit it because it will pop and disappear from sight. And they are cheap. Of course you will have to run down every now and then and replace them.
I think blowing them up would be the hardest part
I like the idea... I wonder how'd they hold up against the winds in Pawnee though... I'll try some next time out