My fish love hornwort, although many people consider it a weed... it overgrows your tank in a couple months. But it is low maintenance & easily removed once you find plants you like better. (unlike some varieties of duckweed...!)
Another easy plant is anacharis, if you plant a wall of them, it will provide plenty of hiding space, especially for fry. These can also be left floating, but I prefer the organization of planting them.
I would also recommend amazon swords, schooling fish like to play hide & seek in the leaves.