PVC marshmallow gun not enough? Fine. Here's a way to make a PVC air rocket launcher.
You'll need a soda bottle - 2 liter size works best but we have done it with a 3 liter with some duct tape. A 2 liter slides right on the 1/2" PVC and doesn't need to be taped.
This is about 3 to 4 feet of 1/2" PVC. It's connected to ...
... a 90 degree electrical PVC joint which is connected to...
... a two foot section of pipe that fits into a cross piece. Two 45 degree electrical PVC joints attach to the cross piece on either side with a two foot PVC pipe going down to the floor to keep the launcher standing up. My husband put two plugs in the side pipes to keep the air in the main pipe and not venting downward. End caps on the side pipes would probably work too. Another 3 or 4 foot section goes in the top of the cross piece.
Okay, now for the rockets. Find some old magazines. (The glossy paper works better than regular paper - less porous.) Undo the staples that hold the magazine pages together. Roll them up starting with one of the longer sides of the page. It helps to have a piece of 1/2" PVC to roll the paper on. You want the paper snug but still able to slide off the PVC. Tape the paper roll.
Stick some plastic bag bits in one end and tape shut.
Slip the rocket on the top of the launcher. Stomp down on the middle of the 2 liter bottle and off it goes! If you stomp on the end of the bottle, the plastic can break. If everyone has their own bottle, no one has to share germs when blowing the bottle back up for the next launch. We didn't put fins on the rockets, but I suppose you could to experiment. This launcher can put the rockets up about 20 feet in the air. We tried it in our church basketball court and hit the ceiling every time. One of these days we'll try it outside and see how high they can go.