Take a weed sprayer and attach to it a small bag (made like a BLOOD PACKET; check out the gore section.)Fill the packet with a damp mix of talc, sand, coloring, anything that hardens up brittle on drying, aim, and fire. Then, on cue, squeeze the valve trigger, the air rushes into the bag, and it blows up spectacularly.
Another good thing to do is just get some chalk and shoot it at the wall with a slingshot.
Also, you may want to check out the ground hits section for some ideas.
