If your tank is small and not really good for other fish, try looking at freshwater invertibrates. Such as snails, clams, mussles, and shrimps.
Inverts will give your betta cleaner water, and something to have in the tank that is living. When thinking about ghost shrimps, always expect the betta to kill and eat it. They can be small enough that the betta ma take it for food and eat it. Nevertheless, they can be used for foods as well as cleaning crew.
Again shrimps like ghost shrimps might be eatten. But conside ghost shrimp and red cherry shrimps. Both are become more popular and easy to care for. They will molt so remove extra skin if you see any. They will help keep the bettas tank clean. When you remove the shrimp from water, be aware. They can and will jump very far.
Snails, Personally i am not much of a snail fan, But apple snails and ramhorn snails seem to be common. They can exit the water so keep a good lid on. I dont know very much about snails nor do i wish to learn, but that is just me. So you might like to try a snail in your betta tank.
Clams, Love them. I just started to get golden clams. These guys i though would be very hard to keep, but they are quite the opposite. Mines do just fine in gravel and in larger betta tanks. They can move themselves with the lip that can come out, and they will feed/ clean your water with the tube that comes out. They are not harmful to your betta and help clean the water. Remember they are animals so dont remove them from the water for too long.
Fresh water mussles. I have seen these in petstores, never had a chance to try them out yet, but they cant be much more difficult than clams. Probably great for larger betta tanks with moving water.
Crayfish. I would probably say no, I have had a few crayfish in the past, and they have never been friendly with me. When i experimented with bettas, the bettas was kill and the fins were eatten. I would try and stray away from crayfish.
Frogs, ADF or african dwarf frogs are not inverts, but good forr betta tanks. Your betta may be alittle aggressive for the frog, but they will make good companions. ADFs stay small, but will usually eat only worms. Stay away from african clawed frogs. In petstores these are often larger and albino. These frogs will kill fish and eat a betta.