What about tiger barbs?? I know they don't get very big and you could probably have a couple in there. Maybe dwarf platies??
I just have a betta in mine. He does really well in it. Because bettas breathe air you don't have to go to all the expense of getting an air pump, air line, etc. He currently has a columbian ramshorn snail buddy who keeps the place tidy. Bettas have alot of personality. Mine know me by sight and will come up to the surface to "greet" me and see if I have any treats for him.