maybe a bubbler? My old betta used to ride bubbles
Also, don't use plastic plants as I've heard those can tear a betta's long fins. I also have a terracotta flower pot as a cave for my betta. He doesn't really use it unless he's scared. Then he hides in it like a dog in a doghouse They're too cute.
if you need to have fake plants i would go with a corkscrew plant (i think they are only in sizes that are too big) or red lidwiga, and i know that they come in 6", and that should be small enough. other than that, any silk plants will work