When I was a kid, we would save light bulbs for months and then go to our "dump" and shoot light bulbs - both regular screw-ins and long flourescent - we tossed into the brush-end of the dump. The game was longest run w/out a miss; at a miss, you had to switch gun to new shooter.
We would save .22 empties and shoot them with our BB guns at 10-15 feet. Now THAT'S tough!!!
For bullet stoppage, take 6" of newspaper and tie loosly - as though you had 10" high stacks. Place into a pan of water (like a dishpan) overnight. The paper should swell until it pretty well fills up the string. While the papers are soaking, make a box out of 1x10s just big enough that you can stand the papers in it on-edge. Tomorrow, stand the soaked papers on end. Make the stack as deep as needed. That'll stop bullets. This was in SHOOTING TIMES a couple years ago.
For completely biodegradable but explosive targets, find a field that was recently disked up and is full of dirt clods. They explode in nice puffs of dirt, and there's absolutely zero cleanup.
No joke...we LOVED shooting cowpies (after the cows moved elsewhere). YOu get a nice, satisfying "whop!" when it hit, a little spray, and the satisfaction that we were helping the process of bio-degradation. And it gave new meaning to "when the SHTF" <GRIN>
Necco candy wafers are great at short range (they're about the size of a quarter); a roll is about fifty cents at the dollar store.
Milk jug lids are challenging...and free if you buy milk!
Have fun...be creative...be safe!!!
Q