Try stacking some pieces of cork bark in a corner. Moss in and around the half log. Extra half logs. A piece of wood or stacked stones they can climb on - tortoises in the store find creative ways to climb on things.
Try silly things for them to check out, just as enrichment. Paper bags, cardboard boxes with food in them, balls they can push around - people under estimate how curious and silly tortoises can be. Don't leave anything in there that might be harmful for them to eat, and remove the "toys" when you're not around to watch them.
I put big chunks of cottonwood bark in my enclosures. My torts climb on or dig under, by turns. They also sometimes gnaw on it, though I've not seen them bite off anything larger than a crumb.