1. an exercise bike. This should be easy. People leave them out on the sidewalk for free all the time.
2. A small air compressor head. Also not hard. Worse come to worse, buy one at Harbor Freight. Ask your buddies. Maybe one of them has an old one they upgraded. While you're at H.F. pick up one of their really cheap cordless electric drills.
3. Hardware. 8 feet of 2 in PVC pipe, 2 elbows, one 1 inch lawn sprinkler valve (24VAC) and fittings to tie them all together.
Cut the 2 in PVC in half. One gets fitted with the 2 elbows, the other with adapters to screw into 1 in NPT which the valve has. Assemble as shown in first picture. Note that the valve scr3ews into place. This is important.
Tear the exercise bike down and remove whatever provided the resistance before. Or leave it in and route the chain out the back. Mount the compressor there and hook it up to the pedals with the chain. You can probably get the small sprocket off the bike. When hooking this up, notice that I put a small lathe chuck on the compressor. You'll want some kind of large flywheel on there otherwise the pedaling becomes very "bumpy".
I put a gage on mine which turned out to be a really good idea. The first one I found was a 0-65 psi unit. Turns out that this setup can be pedaled up to 65 psi in just under 2 minutes, and that's plenty of power. This part is optional as most sprinkler valves have a manual over ride on them but why not? Tale the drill apart and sell the motor to your buddy who's making a fighting robot. Charge him the price of the drill. Cut the drill housing as shown and clamp it to the gun. Route the wires to the sprinkler valve. Yes, 24VAC valves will work just fine on 12 VDC. Or 18, whatever.
The 2 inch barrel will fit an empty caulk tube with a 3/8" x 2" bolt and nut pushed through the spout for weight. These will fly like lawn darts up to 125 yards at 65 psi. The barrel is a screw-on remember? 1-1/2" pipe is standard spud gun. 1 inch fits those shotgun shell lawn darts featured here earlier. Etc, etc. The shotgun shell darts will go 175 yards if fired up steeply.
. At 30 psi, 4 crabapples are fun too.
One other thing. This device is a sovereign cure for adolescent obesity. Give the kids a caulk tube and when the first one shoots it, all the others run out to retrieve it. The one who brings it back goes next. After a couple hours of this they're all tired out and well exercised.