I built a desert terrarium a few months ago for a plated lizard. So far, the best plants seem to be hawortia and aloe, at least out of the ones I've tried, though aloe may be a little spiny around the edges. I have a mixture of 2/3 sand and 1/3 regualr potting soil (peat/perlite) in mine.
Lithops would look great, except they would be extremely hard to keep from rotting in a terrarium. Any kind of sedum would likelly be good (I grow sedum morganianum, donkey's tail, which is a very easy plant that would likely do well) Christmas cactus is also a possibility, and possibly also kalanchoe.
Good luck.