Keeping them curious and active is important. I rescued an adult eastern box. I made sure to give her lots of various veggies, including natural ones. I also gave her a dead mouse every-so-often, she was in an inside enclosure. It takes a full day of warm heat to make a mouse start smelling bad (bloat). But the tort will probably find it quickly. If not, take it out after a few hours and throw the mouse in the freezer. They can be refrozen twice before they are too rotted to give.
If you just got your little guy, let him/her get settled into the cage before you start adding stuff. Don't want to stress it out with new stuff. Hey, a food to try--organic wheat grass (aka cat-grass). Replicates a lawn to munch on.
Oh, you can also give worms, night crawlers, "mousies" (it's a bug larva).
Worm Box
Another idea...get some potting soil, bake it in the oven at 400 for an hour and let cool (that will kill any bad micro-things). Then put it in a plastic box (about an 8" x 8" x 2 " deep), add water to make it kinda muddy. Then add worms and/or nightcrawlers. Replicates a square of earth that they can rout for worms in.
My Eastern laid a clutch of eggs, half were sterile and the other half went bad. She went to a school for the fall and then to a rehabber for winter, and released the next spring. If I hadn't rescued her, she would have ended up as a dog toy or a street hockey puck (my neighborhood was not nice for such critters.)
Barb