Check the local pet store for hermit crab caves. They have some small plastic caves just big enough to fit in that tank. No sharp edges or holes where he can get stuck in either. Or plants, mine loves hiding in the leaves and resting.
I'd get a few earthenware pots. If there are holes in the bottom make sure to stop them up so she doesn't hurt herself. You can also use glass jars - I'd boil them before adding. More plants maybe? Hamster tubes are fun too, and PVC pipes.
/ Quill (Brown Tabby and White Pancake Monster)[/FONT]