Consider, propane is 1.5x denser than air. So, find an underground sewer - run a hose down a pipe to the sewer and let the propane tank run for 45 seconds. Wait another 30 seconds for the fumes to settle, then drop a firecracker down the pipe.
Nice deep underground ‘Boom’. Cheap, probably repeatable, loads of fun.
Here you go! No propane required just regular sewer gas.
http://www.youtube.com/watch?v=4lprje7upq4