What would be the safest, not-cheesy way to level grind in a sand-box like setting?

[Maat Mons]

Under this scenario, I'll... echo myself from the last (and first!) time I heard of the idea of starting a simulation at 0: my money's on the were-rats. When you combine their high DR and low ECL with an ecological niche with little real competition, they're one of the ideal species to successfully develop real power.

Lycanthropolis is inevitable!

Deities are another decent choice as far as "likely to still exist"; the problem is, with their 20 Outsider HD and Epic ECL, they should have a doozy of a time leveling.

Every deity can cast all spells for domains they offer as spell-like abilities at-will. Make sure you offer domains that include Wish, Miracle, and Shapechange. Having those three SLA's at-will makes being stuck at low levels much less onerous.