you could keep any of the smaller scorpionfish/waspfish in it. they are substrate dwellers for the most part, and are definitely cool fish, and won't really break your pocketbook.
a group of leaf scorps (they come in different colors), pretty much any waspfish...you can even mix them.
if you went brackish, you could do an awesome indian mudskipper tank. they stay small (~4") and can be kept in groups. they do need a "beach" tho.