Once upon a time, an idea was had. (I wish I could find where)
This idea was simple. This idea was ridiculous. This idea was glorious...
...Nitroglycerine Melons.
Nitroglycerine* Melons.
Growable explosives. Find a nitroglycerine seed in a loot chest, plant it on a magma block (the melon will also only grow onto magma) and bam...farm-able TNT. Just light the melon like you would any other explosive. (Flint and steel, redstone, flame arrow...) Have fun!
*Contains no actual nitroglycerine. Do not attempt medical use. Does contain explosives, handle with caution.

