This is a cool build. Basically they author uses PVC pipe to build a vacuum chamber to use as a cannon. This would be a cool build with kids.
http://www.instructables.com/id/Large-Bore-Vacuum-Cannon/?ALLSTEPS
Oh the author has another nice write-up on building a vacuum pump from PVC.
http://www.instructables.com/id/Make-a-High-Volume-Manual-Vacuum-Pump/