Definitely suggest a surface plant. Bettas like diffused light and some protection to hide under. They and the surface plants prefer lower current, so the HOB filter you have might be a bit much, unless you can adjust it. Their delicate, long fins can be damaged by strong filters and they are fairly slow swimmers, so it can be a nuisance for them to deal with the current.
Some good surface plants are water lettuce, frogbit, salvinia minima and duckweed. Just make sure none of them are illegal in your state. I sorted them by growth speed with duckweed being the most aggressive.
Bettas also enjoy a good moss like java, christmas or marimo balls. I definitely recommend something with some nice size for the betta to lazily hang around. Java fern, anubias or amazon sword are good choices. I can't exactly tell what you have in there now, sorry!
You're on your way to a very happy betta.