Sand or keep it bare. Gravel and stones that the turtle can swallow will cause impaction and maybe death. They will sometimes eat the sand too but that will pass harmlessly.
Large rocks will work too but I find that stuff gets trapped between them and its hard to clean and if they are light enough the turtle will knock them around and it may scratch or break the glass.
Plants are hit and miss, try something tough like java fern. Mine keeps tearing parts of even my silk plants. I have found that marimo balls work. My turtle took a big bite of one, spat it out and never touched marimos again.