A demonstration for the pyromaniac in all of us.
Fill a balloon with hydrogen – this is only realistically possible from a cylinder. Use some thread or string to tie the balloon to a weight or something to stop it floating up to the ceiling. Then ignite the balloon using a splint attached to a metre ruler.
It will explode with a very satisfying boom. Don NOT attempt to do this with hydrogen/air or hydrogen/oxygen mixtures as the detonation could be damaging.