Bed-a-Beast should be perfect. If you want, you can press it down firmly and compress it so that it's more like a ground covering than loose soil. It's also harder for them to eat it mistakenly like that.
For plants, anything that is safe... it can be a lot of things, although just do a Google search for toxicity of any plants you do choose. Remember that when you buy a new plant, they're very likely to have pesticides on them and the plant has to be re-planted into a substrate with no fertilizers or pesticides for about six weeks before being put into the tank to grow out the pesticides if they have been collected into the plant. If you have the plant already at home and it you don't put pesticides, you don't have to do that. Also, places that sell plants for vivariums aren't going to put pesticides in there.
Hibiscus would be a good choice, but it needs a lot of light and water. Some herbs, like oregano, work well. Bromeliads, certain pothos, most ficuses (although I'm not sure how well it would work in a terrarium), etc. would probably work well.
I, personally, go to Home Depot and bring home inexpensive plants I feel are safe. Then I check up on all of the plants and only use the ones that are, and just keep (or you could return) the ones that aren't.
-----
Got hips like Cinderella / Must be having a good shame / Talking sweet about nothing / Cookie I think you're Tame 