You could have a huge school of cory cats! I think cories are adorable. Tons of colorful snails, definitely; you'll want some tank cleaners. Shrimp are an obvious choice.
You could probably get away with guppies if it's well-planted enough. Or barbs, danios, tetras, that sort of thing, as long as you give them enough cover. But you probably don't want to stock as much of anything that bites fins as you normally would, because of the betta.
If you wanted, that would be a really good tank for a sorority. That's another option.
Want to see what you do with this!