There are many fish suitable for a tiny tank like that but they are not common in your local fish shop because they can be unimpressive and a bit hard to spot when they are in a tank next to a cichlid tank or even a guppy tank. Some nice candidates include Heterandria formosa
black chins
endlers
golden teddies
and all manner of shrimp and similar small things.
You could even get away with a few pygmy cories like this one following a Heterandria (C. pygmaeus)
or C. habrosus
or C. hastatus
Note: all of the fish in these pictures are adults except some of the endlers. It is impossible to have only adult endlers in a tank for more than a few weeks.