I would NOT try male guppies, serpae tetra, many barbs, or gouramis, of course there are no 100% guarantees in the fish world when it comes to how fish will get along. Those few in particular have been pretty much proven to be a disaster waiting to happen.
You could try something like Chili Rasboras. I've known them to be peaceful and active, but not nippy, which is EXTREMELY important when choosing Betta tank mates. I've also heard that Dwarf Frogs can coexist peacefully in a 10 gallon tank with Bettas. Hopefully that gives you a starting point!