I found some "critter litter" in the shed today, it uses paper fibers in a cylinder shape. I didn't even know we had it, but it says it doesn't have any cedar or pine oils. The bag looks like a mouse or something might had chewed it open some.

This stuff would be a good substrate? It would probably keep more moisture than rocks. It also says it has been heated to kill any bacteria. Non-abrasive, and it's safe if swallowed.