A sword plant could be a good addition. There's a ton of different species, I've got an Ozelot Sword in my tank that I really like the look of (Oval shaped leaves with black spots). Depending on how strong your light is, they can grow short stems and look bushy under lower lighting, or grow long stems and long leaves and reach straight for the top of your tank!
Stem plants would be a good option to add in the corners/back of the tank, as well. They generally grow quickly and don't require any special treatment. In my tank I have water wisteria, red ludwigia, temple hygro and green loydiella. When they start getting tall, you can cut the tops or side branches and float/replant them for quick coverage and propagation.