Maybe a pair of honey or sparkling gouramis with some sort of schooling fish, tetras maybe. Some nice plants and driftwood. Or a pair of rams (always wanted some of those!) with some schooling fish. Or you could do a species aquarium. Oh wait, you like bettas right? Maybe get some wilds...that would be really neat!
Have you thought of going with live plants? You could get some low-light ones like crypts, anubias, and java fern/moss (that is what I have in my tank and it is growing in well despite having to bleach dip them) and it really shouldn't cost too much. I use flourish excel, API root tabs, and a 15 watt 6700K bulb on my 15 gallon and it is doing well.