One substrate that works well is a combination of potting soil, sand, leaf mulch and sphagnum moss. When misted every day, it holds humidity well. Just a small cardboard box will work fine, but a variety of logs and caves are sold at many pet stores. You should also make the substrate deep enough for the turtle to burrow completely under. All box turtles need high humidity.
Dry air can cause eye irritation and even respiratory illness.
It can literally kill a box turtle. They burrow into moist soil and come out at dawn and dusk when dew moistens everything at ground level. They become highly active in rain. Thoroughly mist the terrarium every day. Your turtle is likely to be most eager to eat right after being misted.