Rice and say Azolla or duckweed for the floating plant might be interesting.
Azolla will fix N2 gas and turn red under low Nitrogen levels.
Rice paddies are generally fertilized with NH3 liquid injected into the soil, and the soil is flooded and fairly anoxic, so the bacteria does not convert the NH4=> NO3.
NO3 is will not stick to the soil and will wash away, infiltrate into ground water etc, whereas the NH4 will stay put and bind to soil.
So you could have a liquid system using the duckweed and Azolla and a soil based system using Rice(or some other emergent aquatic plant).
Azolla is good and useful as a feed for both livestock(mostly Chicken feed) and human consumption. The color change under N limitation is also good for displays at 4H programs.
Rice is obviously a good crop and loves NH3.
You could also see how well say the Rotala(weed) and Rice(crop) does together under different NH3 concentrations in small pots.