I find that a 10 gallon divided with 2 Bettas is a great setup. Especially with a male and female (as long as you don't intend to mate them).
They really play off of each other and flirt and whatnot. I just put Rex in a 10 gallon divided with Azazel and it's too cute. When he sees her he will flare at her and swim away. She will just sit there thinking, "What just happened here?"
They are such awesome fish!