I used to have a few. I used a lot of sand so that he had around 4" of water in the deep part, then a sand slope, then an area that was wet sand. There was no divider or anything similar, and the crab would make finger-width burrows in the wet sand. No rocks, just a few random seashells. I'd suggest making the dry side nice and wet so that the crabs can make burrows.
Oh, just to warn you; I ended up with mosquito larvae in there. Scooped them out and made them fish food. Keep an eye out for skeeter larvae, and if you see them, just swish a net through there and dump the buggers into a tank with a fish. You've got a week or two before they turn into adults, so they're great fish food.