You mentioned floating plants, and bettas do love cover. Red root floaters or dwarf water lettuce would both be great, although you might have to drop the water level a bit to allow room for the leaves. At one point, I'd left several rotala stems floating after a rescape, and they did surprisingly well. It's not typical, I know, to leave them like adrift like that but they just sent out roots all along the stems — and really liked being close to the light. Just an idea.