The dilemma is that you now have a balloon full of "some gas" and some spent solid, either of which might be hazardous to ... I assume children, who are the only people who really enjoy balloons that much, but YMMV. What you want is the smallest amount of the most harmless substance, to create the most amount of gas, and fairly rapidly too. We have a term for such substances ... explosives. So anything that works in a tiny amount in your soft balloon can be dangerous in large amounts in a rigid container.