Good ideas here! I'll have to try the colored ice some day.
My family has a property in Indiana where I shoot .22s sometimes, nothing but corn fields and woods behind it for over a mile and a half plus I have a very thick wooden backstop I constructed. Lots of walnut trees on the property, my dad says he collected a bunch of walnuts to use as targets so I'll have to give that a try next time I'm out there. Don't know if they'll explode or just go flying, we'll see.