Put a "Buck" target out at about 20-25 yards or so. Then put a "Doe" target right in front of the buck, partially or mostly obscuring the vitals of the buck. Then post a sign at the stake saying: "BUCK ONLY! Hit the doe, lose 5 pts. Wound the buck and lose 3 points. Refuse to shoot, lose 10 points" or some variation on that idea. You can score it any way you want to.
Theme targets: Have a wolf target facing down 3 javelinas on, say, lane #5. Then have a cougar or bobcat target and a cardboard fiddle on lane #6 followed by a "cow elk" slightly elevated somehow with a foam moon under her on lane #7...and so on. The possibilities are endless.
Make it an "ETHICAL SHOOT" - Score 10, 8, & -5 for a wound(outside the kill zone) or 5, 3, & -2 for a wound (outside the kill zone). Have 1 or 2 extra animals/targets oriented in a questionable or risky circumstance and reward the shooters who DO NOT engage.
Give certain species "protected status" , say canines and felines, and place them in front of or partially obscuring kill zones of "legal game" then dock shooters points for hitting the "endangered species."
Put an iron mask or plate with a 6 inch hole in it in front of the kill zone of a target. Shooters have to shoot through the hole or they will break or damage an arrow...
I really like the water bag or 2 litre bottles of water on the pulley thing mentioned above, too.
Just ideas.