I used to make Hydrogen by mixing lye and water and adding aluminum foil (when I wasn't making my own black gunpowder for firecrackers).
Another trick: to fill a hatbox, fitted with a short tube through the lid, with natural gas (propane would probably work OK for this too). Ignite the gas at the tube. Flame burns at the tube awhile, then seems to go out. Ten minutes or so later, box goes kaboom. (Hatboxes have lids that slip over the bottom part, so exploding box only shoots lid up into the air. At least that's how it was described in the "magic book")