I agree with the suggestion for the betta log. It's great because your betta can rest near the surface and won't have to travel all the way up the tank for air, which can be stressful, especially when it's trying to rest.
Your tank will probably look quite filled out after adding the log, because the minimum size requirement is 2 gallons. ;)
If you still feel it looks empty, then maybe you could add some java moss in the background...they keep ammonia levels down (but will take over your tank quickly if not trimmed regularly!).