I'd go with a shrimp colony, or a nerite snail since they can't reproduce in fresh water (most snails can reproduce with themselves.. O.O)
For african dwarf frogs, you'd need 2, since they like company. They'd need hiding spots also, and you'd have to be pretty dedicated to them. Aggression is possible both ways, betta could bite off frog toes, frogs might be tempted to chase a bright betta and nip fins. Also they are very slow at food, as Myates said, so you'd most likely have to shove the food right infront of their faces to make sure your betta doesn't eat all the food. A good choice for food is the HBH frog pellets, available at petsmart, with supplements of bloodworms. All in all, unless you have your heart set on them, I wouldn't do it ;)