I was as surprised as the people in this video (FF to 0:45) at how well the 4M Rocket lives up to its claims of blasting over 30 feet in the air. I didn't yelp like the lady filming but I did laugh at her and, as a result, dribbled some coffee on my shirt.
From 4M Products comes this kid-friendly and grownup-worthy backyard Water Rocket Kit. Builders Jr. and Sr. alike learn the physics and mechanics of rocket construction, with the sweet payoff of bubbling up, building pressure, and blasting off in a spray of H2smOke. The 4M kit includes all required parts and instructions for assembly, less the pump. (Even though one area of the Water Rocket listing says a pump is included, you'll need to supply your own. Any bicycle pump should do.)