Right now this is what I'm thinking:
fill it halfway with soil
plant grass in it.
let grass grow tall
insert gerbil or hamster (whichever likes to tunnel more)
I think that set up would make a great play space for a gerbil or hamster. It would be too hard to keep adequately clean for a permanent home, though. How big is the tank? Bigger than a breadbox? Smaller than a bathtub?
How about using it for an herb garden?