I've been getting into shooting at wasps with salt, but other than that, any water container full to the brim makes it pretty big boom when you hit it. You could even add those orange target stickers if you wanted to be a little more accurate, I usually put a drop of orange spray paint as the center. A drop or two of food coloring in the water makes it more fun as well, for some reason, blue looks really cool when it "explodes".
Old hanging pieces of metal that swing around and ring when struck work great too, and at muzzleloading velocities, at the metal doesn't need to be very expensive or thick.
Old hanging pieces of metal that swing around and ring when struck work great too, and at muzzleloading velocities, at the metal doesn't need to be very expensive or thick.