Bettas love them some floating plants. Frogbit is good, but mine really seems to groove on the ball of riccia I have floating in the tank. If you have the room, a floating watersprite would make a good plant for a betta, but most betta tanks are small enough that watersprite will choke the surface and send roots to the bottom.
For the record, my betta tanks have had Java moss, java fern, Hygro polysperma and difformis, Rotala indica, Anubias nana, and various cryptocorynes. The one I am working on now has glosso, dwarf sags, Rotala indica, Ambuila aquatica, and Cryptocoryne wendtii, but that's a bit higher tech than you're looking for.