One of my favorite things to do with my granddaughters is to use pneumatics to lob things around the yard. This is a great build that is very affordable and can be made by anyone. All you need is a pair of adjustable pliers, a pipe wrench, teflon tape and ideally a bench-mounted vise grip. You will need a small pancake compressor capable of producing 90 psi.
The general idea is to launch a Stomp Rocket several feet into the air by sliding the rocket over the barrel. The slide valve is the trigger mechanism. You move it forward to charge the chamber with air and slide it backward to dump the air very quickly. The result is a sudden burst of air.
I do not recommend operating this component at any greater than 40-60 psi when children are present. Yes, it is a foam rocket but all it takes is one careless move or leave it unattended to do serious injury to the eyes or throat. We primarily use these projects to “lob” these rockets at baskets. At times we shoot them just to see how far they go and the girls chase after them…we do not let them try and catch them in the air. Not safe.
This same configuration can be used to do a multitude of things by swapping out the barrel. I have installed 4″ barrels and launched foam torpedo footballs about 150 yards. I have swapped it out with another barrel to shoot nerf darts and they quite seriously disappear from sight. That is really a BLAST!