@ The Old Spouse
You can get some cryptocoryne balansae or aponogeton crispus for the back or left side, easy growers and not so boring like vallis although I like vallis too. They'll give a bit of height too and maybe shade once they grow to the surface. If you have ottos, they really love those sorts of plants, especially if they are moved by the flow.
Thanks for the suggestion, snazy. I have to say, though, that I'm really liking my tank now. It's the only one that is pest algae-free, and I rather like the balance. I now have 11 tiny zebra danios taking up the upper third of the water column, and that's what I was looking for. Also, the cory love to hang out in that "bare" spot in the back middle.
Honestly, with the algae problems in my other tanks, I'm concerned about doing much to this tank that might upset it's balance! I do like the idea of the aponogeton crispus though.
I like the tank the way it is too. Adding too many plants will upset the balance in a low tech tank.
You can try crinum calamistratum. It's similar to aponogeton crispus in a way but has way narrower and very curly leaves. It grows slowly so won't take up much nutritients at once and won't overshadow anything due to the thin leaves but will make it interesting for the fish. I really like that plant. I have one myself.