I am still learning the plants that fit best in a terrarium so I'm not much help there, however I can offer some advise on the crabs, if you decide to go that way for critters.
As far as the crabs go - some like to burrow. I have red chilie crabs that burrow. In my land portion I just have a potting soil mix (minus the vermiculite/perlite). The crabs will dig as far as you let them, and in some cases without a trace. (I pulled my tank apart a couple of months ago as I thought that my crabs escaped. I left the land mass sitting on a desk thinking that nothing was in it. The next morning when I went into my room I found a crab crawling across the floor. lol
Crabs do need a mix of water and land. If you have enough water some small fish are a nice addition. The crabs shouldn't bother them.
I have also noticed that the crabs will munch on most plants.
Good luck. Post some pictures when you can of what you've set up.