First are floating plants. Anything to reduce that light and make the betta more comfortable; betta love cruising among floating plants and branches. Even a cheap stem plant allowed to float will suffice.
In that environment the Java Moss would be quite nice. I just went in to my fishroom and took a photo of my 10g, attached below. There is a chunk of wood with JM on the right side, you can see how it grows up from the wood. I pulled out a handful a week or so ago.
The plants show what can be done with odds and sods; the floaters are all Echinodorus bleherae daughter plants culled from the 115g to thin them out a bit [I have dozens if anyone wants to come up and get some
]. There are daughter plants of E. tenellus and one or two of the red leaf swords in the sand. This tank was just thrown together, no filter, no light, just a heater. I have 7 Corydoras pygmaeus and 9 Boraras brigittae. Plus a puffer (not sure what species) and a couple shrimp (no idea which); these "arrived" in the bag with the C. pygmaeus from one of the local fish stores whose owner will deliver, so I take whatever she catches in the net.
Oh, and definitely lots of snails.