For protein you could also try eggplants!
Heads up: 21 grow spots doesn't necessarily mean 21 mature heads of lettuce will fit in there at the same time - what's your spacing like? Might want to rotate seedlings on one side, more mature plants on the other, spaced out in a way that the more mature plants don't block light to the younger shorter plants.
You can do tomatos, bell peppers, and lettuce at the same time. The tomatos and bell peppers will take longer though, so you might get 2 rounds of lettuce in before you get fruit.
I haven't tried root vegetables other than radish, though radishes work GREAT! I've been curious to try potatos though.
My hunch is you can grow anything with a little bit of hacking on the plant support system (i.e. stringing up tomato branches so it doesnt fall) and maybe swithing out the growing substrate (maybe root vegetables would work better in a hydroponic system that uses expanded clay pebbles instead of the floating pink tray).
Curious to hear what you try out!