This DIY catapult is simple to make from materials you can fish out of your recycling bin. It was a fun indoor activity for us on a rainy Friday afternoon.
I first attached a piece of cardboard to foam core board with some duct tape. Foam core board is sometimes used in packaging. If you don't have any, search around for some thicker board or try several layers of cardboard. I taped a stiff cardboard tube (must be sturdy, a regular paper towel role is too flimsy) at the midpoint. I glued a milk bottle top to one end.
"This is fun!, he said over and over while I sneaked away to enjoy my cup of tea in peace (sort of).
At one point, I said "Bombs Away!" when he shot another ball across the room. Since Kiddo has probably not heard that expression before he started shouting what Mom thought was "Barnes and Nobles!", but was actually, "Barnes N No Balls!"