Almost every year that I have taught Physics, I have used the Water Rocket activity. They are relatively cheap and easy to build and launch and it never fails to inspire awe. Here are the instructions I created from experience and various sources (link).
I have used various water rocket launchers and they each have their drawbacks. This time, I bought the launcher from this site. It worked perfectly and never had an issue in hundreds of launches.There are multiple options, you can either buy the launcher, buy a kit at a discounted price, or build it yourself for free. His customer service was awesome and to be willing to give the plans away for free is much appreciated to teachers not afraid to DIY.
As long as the students play by the safety rules, they are free to be as creative as they like. It is a great introduction or review for trigonometric functions and solving linear equations. Also students loved seeing a little cloud being formed by the pressure release after launching. Lots of great questions asked before, during, and after launch. Just a fun project all around.
