You can do a 2:1 mixture of hydrogen and oxygen by volume, that results in pretty big bangs... something totally different from just filling a balloon with hydrogen gas that burns with air once its ignited.
I'd recommend extreme caution though, with a volume of a couple of liters, you will need ear and eye protection. As a chemistry demo i ignited testtubes filled with this mixture, and those already produce a somewhat scary bang. They dont shatter if you ignite the mixture from the open end, but if you would put in a cork and then ignite the gas somehow, i think they might.