I would experiment with bicarbonate of soda and citric acid.
citric acid comes in liquid form and powdered form.
If you mix the two powders, you'll need to add water to get the reaction.
So what's the reaction?
it releases carbon dioxide.
I would try with a few things to get the boom.
My first idea would be to fill a smallish capsule, one that you can just squeeze open, with bicarb and citric acid then place it inside a water balloon. Fill the balloon with water then tie it up. when you want it to explode you squeeze open the capsule and the reaction will start and cause the balloon to explode