My Webelos Cub Scout Den had a BLAST (literally) doing these 2 liter bottle water rockets. The way you do these is you fill the 2 liter bottle about 1/3 full of water, put it upside down on the base, pump it up, and pull the pin out (attached with the blue twine), and watch it shoot off. They did it the whole hour and couldn't get enough of it.
They loved experimenting, and doing different amounts of water, and different amount of PSI by pumping it different amounts of times. It was easy for them to do by themselves, and you can see by looking at the pictures that the rockets go incredibly high.
This would be a fun idea for a Pack Meeting. The boys could come with their bottles decorated. As long as they don't make any holes in them.