I would suggest Anubias, small Echinororus and floating plants you can allow to flow over the surface and provide shade. Hornwort works well in my hard-ish water. I anchor; can't plant as Hornwort does not have viable roots.
I know someone else said to stay away from water wisteria, but I'd have to disagree. I have it in two of my five gallons and it grows like crazy, a really beautiful plant, and when it gets too tall you can always cut it back.
Crypts are awesome, and depending on your lighting ludwigia could be great too.
Two easiest plants-java fern and anubias. Don't need to plant in the substrate as they are water column feeders. Hornowort, guppy grass, and anarchis are all good water feeders that you can float and don't require substrate. I also LOVE floaters-just not duckeweed.