About the balloon filled with hydrogen: A balloon filled with only hydrogen will have a flame that must go out in search of oxygen to burn. It still makes quite an impressive fire ball, but if you want a HUGE explosion, fill the balloon with both hydrogen and oxygen. I think twice as much hydrogen as oxygen, but I don't quite remember the ratio.
This way, the oxygen source is located within the balloon, and believe me, you'll be able to hear the difference. It's a boom that will leave your ears ringing and set off some car alarms. It's not as big of a fire ball, because the hydrogen does not need to search for oxygen, but it's a much louder boom that puts out quite a shockwave.