I work at a Very cool Ecology based Summer camp
and for the 4th of July we do Bottle Rockets
all sized bottles, Corks, and Bike pumps for the kids
and sometimes a portable compressor for one or two of the Staffers to show off with (dressed in scuba suits or full foul weather Gear)
we use some natural food colors to make it 'pretty'
the kids run around and around collecting bottles so we can shoot them off again
we start with the same number of bottles we end with
and the kids LOVE it