10 challenges we set a while back - the Materials Challenge. Basically you have to build a water rocket using only PET plastic bottles. You are not allowed to use any glue, tape or any other materials! It was a fun challenge and things didn't quite work out as we planned but we'd like to visit it again to improve a couple of things in order to get more performance.
Because we couldn't use our regular nozzles for the challenge (they are not made of PET), we also built a Clark Cable-tie launcher to launch the rocket. We've also put together a tutorial on how to make the launcher. It fits a standard Gardena release head so it can be used it with our regular launchers.
The tutorial is here: http://www.aircommandrockets.com/construction_9.htm