Better lighting, more substrate and a black background. Emphasis on the black background!
Were you looking for opinions for stocking the tank as well?
If so, lots of lower light plants and one big shockingly beautiful Betta. That would make me realy happy. (I like the colors that come out in fish when the lighting isn't too intense)
As far as Aquascaping is concerned I'd try to keep relativley small plants and rocks up front, unless you want a certain plant or rock to be the centerpiece for the tank. Then keep the larger plants toward the rear to open up the center of the tank and to hide ugly equipment. You'll be able to see your fish more easily with the new layout. If you pull it off nicely you can really create a nice sense of depth, even in a 5 gallon tank.
Wisteria Difformis makes a great space filler plant. Cheap, easy to find and easy to care for. Just make sure to give it enough light.
Good luck!