Betta is the best choice.
Betta, Ghost Shrimp, Snails, Dwarf Frogs and Mountain Minows. Thats about it and I mean it.
i have a 5 gallon and i bought all my fish still sort of in the fry stage, there a lot cheeper and it gives you something else to be wowed about
i got 2 fancy guppies and a ballon belly moly and an oto, i know you guys will all probly say thats way to much, but the guppies had fry and even they matured happily, sure i gave them away, but i think you can deffinatly keep more then just a betta