A crab tank is about the least expensive and easiest to keep tank that I can imagine.
There are a number of crabs that are interesting as well, in addition to the typical hermits used in cleaner crews consider emerald and decorator crabs. All inexpensive and easy to keep critters.
I would set up such a tank with dry (e.g. dead) rock and 1 small piece of live rock then cycle it with a raw shrimp from the grocery story. (Search the forum for how to properly cycle a tank, tons of info on it.) This will give you the ammonia filtration you need and give your crabs plenty of stuff to climb on for a nice presentation.
I have seen your other posts and understand what you're trying to do, this post is a very reasonable question and a tank that can be done on a tight budget.