For fun targets I sometimes use a piece of 2x4 with 1/4" grooves cut lengthwise into each surface. This is just the target holder...
The targets are standard 2" x 2" saltine crackers. They explode when you hit them so you can even tell from a distance whether or not you miss... when you are done, put the target holder 2x4 back in your trunk...
...and the local birds will clean up the cracker mess when you leave... a very green solution eh?
I like the Ketchup packets idea, but they are a little messy. One advantage to the ketchup packets are that if you take your lunch break, when you come back, the backboard will be covered in flies... and you can pick them off with your scoped .22 rifle!
