Sunken Ruins?
Well, you could go with the generic story of an adventurer entering some old temple to find treasure. Still better than no story.
Or maybe a failed archeologist seeking ancient statues to become famous. This is probably a better idea. The main task would be to find a highly valued statue(which you will in the last level) but you could add a side mission which includes six super secret levels which are hard(hard to find, or just a hard room) but optional, each rewarding with 1 piece of the Super Secret statue which gives some bonuses, possibly just a little better then something you might buy in shops. Then, when you have all 6 you combine them into a super statue thingy or something like that.