Explosive Runes on a weapon?

[barakaka]

You could go Geometer and use Glyph of Warding to put a fiery spell on the weapon. Kinda expensive though.

Also pretty expensive and maybe not DM approved:
Go with Alchemist Savant (Magic of Eberron) and create spellvials of like burning hands and Explosive Pack (alchemical item I think in Secrets of Sarlona).