You could make hydrogen and oxygen from water. Make a "U" shaped container from PVC pipe. Drop a bare wire from each pole of a 6 volt lantern battery in each side. It will immediately begin to bubble. Hydrogen out one side, oxygen out the other. Capture the hydrogen in a plastic garbage bag and it will float. Later, after the fair is over, seal the end with tape and attach a self addressed, stamped post card to it with a request to note the location, date and time where it was found. You might be surprised how far it will go. Be sure to lable the bag with warnings that it is FLAMMABLE HYDROGEN.