Crabs are very good at escaping. Have you thought about African Dwarf Frogs? Kind of un-exotic, but if they have a lot of places to hide I think they are more active in shallower enclosures with a larger footprint. In the wild they spend the dry season in basically small sandy puddles or pools so they enjoy wider shallower areas. I would also suggest some snails like Apple Snails. In a set up like that they will actually come out of the water and forage in the shore areas and a little on land. They aren't as good at climbing out of the water so they probably wouldn't escape but you will find them all over the land area. Also, they sometimes dig in to the terrestrial soil and disappear for a few months, I guess hibernating. I had a similar set up on a balcony. For awhile I had apple snails and a Betta in it, but eventually I put in Firebelly toads and I had to add a screen.