A very fun set of books
I just finished "Ready the Cannons," by William Gurstelle, who also has released "Backyard Ballistics" and "Defending Your Castle." "Ready the Cannons" lists a great number of projects, mostly using PVC piping and BBQ igniters. The beverage bottle bazooka (small soda bottle, vapor from rubbing alcohol, and launch tube), a hydro swivel water gun, a carbide cannon (I remember when those were sold in kid's comic books, they made a heck of a blast while not being fixable with a projectile, and so were not legally guns). A potato gun with a 10 foot barrel, to see how high you can fire its load. A device that accelerates ping-pong balls to 300 mph.
Seems like a nice Christmas book (or perhaps one to get early so as to build a few for presents). It and the two other books are available on Amazon.