Well for true floaters I would say Frogbit. It will grow fairly fast but can easily be removed without much work. I don't know the light requirements for floating plants though. I know the w/g rules doesn't follow the same for them but they do need light. Red Root Floater and Watter lettuce will cover the tank eventually with smaller plants and may get large but the larger ones can be removed for the smaller ones. Red Root Floater seems like it has the best hanging root mass if you are looking for places for fish and fry to hide.
The site says alot of them still require higher light but I don't know. I have heard they will grow in as little as 1 w/g because there is no diffusion from the water. No top to the tank might help also because the plants would perform that function.