I personally love water lettuce. The roots grow pretty long but you can trim them if you don't like how that looks. I personally think it looks awesome at the surface.
I've also used brazilian pennywort for a long time. The roots don't grow as long. You have to take them out each water change and trim them up to keep them looking fresh. They grow a little more slowly.
Whatever you do, stay away from duckweed! I got some accidentally in another plant and it just is impossible to get rid of. Rinse plants reallllly thoroughly before adding them to your tank. Learn from me.