Well, there's the usual crew of shade plants: duckweed (recommended if you have a goldfish or vegetarian fish), hornwort, and riccia.
Duckweed, depending on the variant, can have either really short or really long roots. It doesn't add a plant at a time, it multiplies. Similar plants would be things like azolla.
Hornwort is also good, as it tends to form a "mass" rather than send out roots. Same with riccia.
Another thing you could do is just add trimmings from when you trim your plants. I did it for a while with my HC, and it looked pretty cool.
So many fish/plants/inverts to keep, not enough aquaria.