The volcano idea is good. but have it irrupt and have lava monsters come out of it want wander around...., maybe have a quest/deed the whole mud can do... eliminate the lava monsters... and stop the volcano... to stop the volcano.. you need to kill all the lava monsters and a appears..., you push the boulder on top of the volcano and thats the end.
the deed/quest could be "Saved town from Volcano" or something
some monsters could be.
Lava golems, lava bats, lava snakes. 
just an idea.. you could prob improve on it alot.

