Ours is the tray from an old hamster cage, filled with first some lizard sand we had lying around (the hamster rejected it) and then soil. Monty regularly gets the soil caught in his fur, so we have an endless cycle of sweeping it up and having to put new soil in (so if you do do one, I'd recommend putting it somewhere easy to clean behind them!) He occasionally digs in it (more often he digs everywhere but his digging tray), but more importantly, it's usually his favourite place for having a flump in the evenings while we have the television on - it provides a different sort of texture/temperature for him if he wants to relax.