You can do frogbit, although this plant doesn't like a lot of water movement. Right now I have Water Hyacinth in my 55g. You can also use Water sprite, the oakleaf water sprite is my favorite. Water lettuce, hornwort (too messy IMO
, and others can all be used as floaters.
Duckweed is a food source for many fish. I grow it outside in the summer and feed it to the goldfish and koi. Even my angels like a little now and again.