I think variety is key to keeping them stimulated. I have four RTs in a two by four tort table my husband made for them. I wanted to keep it interesting, and because I know they like to climb and burrow tried to give them a habitat with variety of surfaces and materials. We use bed a beast with playground sand mixture as a substrate and try to vary the moisture from one end to the other.
We pried the bark off a piece of large piece of fir firewood, baked in the oven to kill any critters, use it as a ramp up to their shelf lined with ceramic tile. They can bask ontop of their shelf, or burrow underneath. They have a log hide to climb over, sleep in. I have a pile of medium sized river size rocks (washed and soaked in super hot hot water to sanitize) I rearrange these as a small barrier between feeding stations, they power over them in nothing flat. I also have a couple short lengths of branches from my apple tree (sharp points trimmed off and oven heated to kill any critters) I put at random spots to create an added obstacle course, and lastly some large pieces of natural slate for basking. The bonus is I never have to trim claws, and there are plenty of sight breaks if they are not feeling social, several choices of sleeping areas and also three different feeding stations. We also found a large cuttlebone (the types for parrots) sold in bulk at our local pet store, they will not touch it for months and then suddenly I see all kinds of new nibble marks and it is constantly in a different spot.
They live in our home office and it is not uncommon to notice them climbing to highest spots to see what we are doing or begging!