I wouldn't be too concerned with it being too small for a betta like many people here would say, although you may have too much current. Bettas like to have fairly stagnant areas to sit in, so if your filter is giving you decent current in all areas of the tank, the betta could be very uncomfortable.
You could put 2 guppies in there instead of the betta. You could probably get 1 or 2 dwarf platys, but I think some places incorrectly label fish as dwarf platys when they get well over 1" long. You could fit 2 endler's in there if you can find them.
There are some snails you could get, but a mystery snail probably isn't going to cruise around the walls as much as it would in a larger tank with wider walls.
A few shrimp would work, but most of the gost shrimp you'll find aren't going to last very long. They usually just die on their own after a few months (maybe aquarium salt would help that). Cherry red shrimp are good if you can find them.