Hydrogen balloons are actually lighter than helium balloons. However, since they tend to explode, their use in birthday parties is heavily curtailed.
2H2 (g) + O2 (g) → 2H2O (g) + heat
You can create a more powerful balloon, by mixing oxygen and hydrogen together in the balloon. Thus, every H2 molecule is very close to an O2 molecule allowing an even faster oxidation (combustion) event. However, it matters which gas is put into the balloon first, as an explosion can occur spontanously (via Joule expansion).