I think that's a bad idea that could put someone in the hospital. I mean, it is a stinkin' great idea, but a dangerous one.

Garbage bags aren't meant to break upon impact, or in flight. You'd have to devise a way to create an expansion with sufficient force and speed to create an explosion. The problem with that is that most of those would involve dangerous reactions of some sort (the ones that I would use anyway). The safest would probably be dry ice, but if anyone touches any lasting solids... it would be bad...

Honestly I'd forgo the trash bag idea. There is no guarantee they'd break in the first place, even upon impact unless they were really cheap trash bags. And there is no way to guarantee a method of destruction midair either, even using aforementioned reactions, because trashbags (cheap ones or expensive ones) don't expand like balloons, so they would break at one point and water would spill out that way, and some might get trapped depending on the size of the breach. Water balloons work because of how they burst and excess retract ensuring that all the water leaves at once. There are no trash bags like that.

Basically this idea is unreliable at best.

If you wanted to go big like that, why not just make a catapult with a bucket or net, and fill it with as many water balloons as you can muster and launch water balloons en masse. It would be safer and give the same general result. Perhaps you could even use party balloons if you want to go bigger

Easy, cut open a side of the bag, close the gash with duct tape, tie the end of the strip of tape to 20-40 ft of string (Imagine shoe laces) tie the other side of the string to a fire hydrant. (or something heavy)

When bag goes flying, at a certain distance the string rips the strip of duct tape, the bag now have a gash letting stuff fall.

Oh yeah, for fillings, an idea: Used kitty litter