Soda crackers, or graham crackers make good, biodegradable targets for informal games. Use the little 2 1/2" square quarters for short range shots( 25 yds or less) and the full cracker for longer targets. You stand them up by wedging them into cracks in boards, or tree trunks. I suppose you could take "2 by" and rip a slot down the length of the boards to hold the crackers, too. Use two range officers, one to go with the squad of shooters, and keep score, and the other to follow behind and put up new targets to replace those broken. We used to give bonus points if someone shot the center out of a clay target and left the rim behind. O for a miss, 5 for a hit, and 10 for the center hit.
We have shot beer bottle caps, slices of damaged hickory hawk handles, or closet rods(wood), eggs, both empty, and with yolks and whites- and a rotten Egg to take points away from that shooter--
in addition to all kinds of reactive targets to put out on a field. Corn cobs still work. We shot a " Kick the can" game with rifles, giving 1 point for every foot the can moved from its starting spot.
One New Years Day, it was about 10 degrees out, and the river next to the club was frozen more than 8" thick with ice. We shot chunks of ice along the river banks, hanging from tree branches, where the high water mark left the ice pieces suspended several feet above the new ice we walked on. I have shot at knot holes in trees, snags sticking up out of a flooded river, and all manner or rocks, or stumps along trails. Hit or miss.