Aquarium gravel will work, too, but they cannot tunnel through it as easily. Make the other end no deeper than an inch. Create a slope on which they can climb. Add dechloraminator to the water if you use tap water. Next add about one tablespoon of kosher salt or two tablespoons of rock salt to the tank.
Filtration is optional.
You will need to top the tank off with new water every week or so to replace evaporated water, but do not add more salt. The salt will not evaporate at the same rate as the water, and your crabs will still have enough. Crabs are not affectionate pets, but they are quite interesting to watch. The little male waves his giant claw about in a semaphore like fashion when you approach, or when he wants to impress the lady crab. The crabs will dig tunnels in the sand and they may scuttle into them to hide if you're not very stealthy.