Here's something I remember doing as a kid...
Raw materials required:
Matches or lighter
4 empty, opened cans of same size (average size, like the normal canned corn can)
The tennis ball should fit moderately snugly in one of the cans.
First take three of the cans...drill a largish hole (maybe 1 1/2"?) in the base of the can. This will leave a baffle ring around the edges, this is fine. Take the fourth can, and drill a small hole (maybe 1/8") in the side of the can near the base. Then duct tape them tightly together, with the fourth can as the base, the others on top.
Bend the wire hanger so it constitutes a nice stand...mold the main hanger to prop the "cannon" up, and bend the hook straight so it sticks in the ground steadily.
Load the tennis ball into the end of the cannon, it should fit down into that first can. Using an eyedropper, shoot a couple squirts (I really don't remember how much) of rubbing alcohol into the hole in the base. Roll it around for 30 seconds or so, so it covers surfaces on the inside. It will start evaporating within the cannon. With the cannon facing a safe direction in the stand, touch a lighted match or lighter to the hole in the base.
Boom! I had a lot of fun with this when I was younger.