I'm getting an african dwarf frog for mine. They are completely aquatic and apparently get along really well with Bettas.
African dwarf frogs and apple snails may or may not work with bettas. It all depends on the personality of your betta. Some are more aggressive than others and will attack the frogs and snails, others will just ignore them. I had to separate my betta and snail because the betta was too aggressive and kept the snail pinned in the corner. I'm afraid now to add a froggie. If you do get an african dwarf frog, be prepared to rehome him if the betta doesn't like him.