Or, to avoid abuse, make the only type of grenades things that don't do dam,
but instead do other effects...
Flashbang type: stuns
Glop: mudpit-like effect
Gas: hallucinate+epeater type effects
Other effects that could be reasonably safe:
* Light
* CoT
* Rain
* Antimagic field
* Dispell magic
* Drying wind
* Entangle