We planted a variety of plants, at different stages of growth. After a bit of experimentation, cosmos grow extremely well. These were cuttings we put in just a couple weeks ago, and they're already bigger than most of the other plants that have been in there for over a month.
The very young plants did not survive, but the larger ones did. If they're too small, they just get swallowed down into the cups by the bobbing of the water.
Seems like the plants need to be large enough to reach all the way to the bottom of the pots, and still stick out enough.
Going to be interesting to see what it looks like at the end of the season.
We're using LECA as a substrate to keep the plants in there, but I think gravel would probably work as well, or possibly better. (the extra weight would likely help stabilize things)
Random LECA link: https://www.thespruce.com/what-is-leca-and-why-do-houseplant-gurus-love-it-5092763