Use aquarium sand as your substrate. Buy more than you would need to make a normal layer across the tank, and use the left over sand to build up one side. The sand will get waterlogged and compact together, so the beach will stay in place. Make sure you have a gradual slope or some driftwood coming out of the water so the crabs can move in and out easily. If you use plants, they should either be plastic or potted, as the roots won't be able to grow easily in the sand.
