There are plenty of options that may fit there despite some people's protest that it is too small. Bettas are obviously the usual suggestions although the likes of sparkling gouramis, licorice gouramis, dwarf puffers and otos can be considered. Even a trio of male guppies, ember tetras, celestial pearl danios and lampeyes may work there.
If you want to try invertebrates, I suggest starting with red cherry shrimps. For snails, Malaysian trumpet snails, ramshorn snails, pond snails (Lymnaea stagnalis) and bladder snails will obviously work or for exotics, get a nerite snail or two but be sure to supplement them constantly with algae. They will consume algae but anything else will be ignored so consider yourself lucky if yours manage to consume commercial foods and veggies. You could also try mystery snails (Pomacea diffusa). Allow one snail per 2.5g rule as these snails do have a tendency to excrete heavy loads of wastes and can grow to a size similar to a golfball. If you don't want anything else, how about assassin snails (Clea helena)?