Rainbow Swiss Chard has really colorful stems. Start early and transplant the most colorful varieties into the garden. Potato plants are very nice and some varieties grow in about 6 inches of soil. Curly Kale is pretty and can be started early,too, so it would be nice by Aug 15. I also did some Alpine strawberries, which had nice looking plants and kept flowering and bearing right through November. They need to be started very early to get berries. They were very slow to get large enough to transplant. How about some of the colored sages and purple basil?
Good Luck,
Keski

What a nice idea! I taught high school biology in Vermont for 36 years and my students always liked labs dealing with plants. I had root boxes (glass covered with sliding boards) where they could measure and observe root systems of various plants. They took measurements on growth rates of different kinds of plant roots. It was interesting for them to see the mass, pattern, and rate of growth. Also, you might consider inoculating beans with Rhizobium in one of your see-through containers. It's easy to observe nitrogen fixing nodules form on root systems of leguminous plants and therefor how these improve soil nutrition.
Best of luck with the project and please keep us informed.
dave

Dave,

You are out of my league! I would have no clue how to go about getting Rhizobium nor how to inoculate the beans.

But reading about roots gave me another idea - grow a tomato seedling that is quite tall, almost leggy. Find a tall clear container and bury the plant to its leaves, marking where the original root ball was placed. People can see how it forms roots along the entire underground stem.

I have cross-posted this idea in four or five different forums and I am getting fantastic feedback on different ideas.

Mike