Burn flammable blocks.
Water destroys redstone contraptions and rails, also collects wheat.
Pistons can break cacti and some other plants.
Burning flammable stuff (like wood) won't break them. the blocks will be consumed in the process.
I haven't tested the thing with water and rails.
and yes. pistons can beak cacti and sugar canes, I guess.
Water as mentioned will remove torches, redstone, rails and plants, but it sounds like you want to do more than that. Without mods, no there is no way to break blocks without you actively using a tool to do so.
1 -Setting off dynamites (or creepers)
2- Releasing gravity-affected blocks over things like torches or sappling ...
3- Pushing with a piston (any part of the sugar cane, wheat, dragon egg, cacti?)
4- Water will remove torches, redstone, rails and plants