When I was in college we used to amuse ourselves by doing just that, you'd have to use a glass 1/2 liter bottle in a bucket of ice water though because the reaction is highly exothermic. We'd put a balloon over the mouth of the bottle and wait for it to fill up, tie it off, put a piece of string around it soaked in lighter fluid, light it and let it go. Nice airburst.
It's a wonder we didn't all kill ourselves... you just don't think of those things when you're young.
Or.............. dropping a chunk of sodium metal (yeah elemental sodium is a metal) in a sink with a little a little "wetness" in the bottom of it, watch it "flammably" dance around reacting with the H2O to form sodium hydroxide and hydrogen gas. If it was the right size chunk of sodium, the hydrogen forming above it would explode with a bang. Kurt