You could get a school of white cloud mountain minnows, they tend to be less nippy than most schooling fish and though they prefer colder water, they should be fine at the high end of their temperature range. Pygmy corydoras are a given, they need to be kept in schools of at least six, but are very cute and unlikely to illicit any aggression from your betta. Kuhli loaches are also very interesting fish that need to be kept in groups of 6. They are largely nocturnal, but I love watching them, they are very interesting fish. African dwarf frogs are another interesting candidate, they are better off if you can get more than one since they are a social species, although they are notoriously difficult to feed in a community setting.
Please research each fish you consider buying separately--understanding the needs of every species is invaluable to their care.