Pair of mid sized central american cichlids something like a pair of Sajicca or Honduran Red Points, Firemouths and relatives would be an option as well. Or you could do a cool South American Puffer tank
For a tank that size, I would favor a fish that I know already. In my case that is Herotilapia multispinosa, known as rainbow cichlids. They only grow to about 5 inches long so they will do fine in such a small tank. This is a picture of mine but I would not put more than a pair in such a small tank.
Totally depends on what you prefer. The problem with having a pair of fish is you have to have a plan for the young (if you intend to raise them that is).
Perhaps some idea of what you like would help us make suggestions.