For those schools that have a garden, this is a great idea:http://www.pbs.org/wgbh/nova/satoyama/
We currently have two gardens at our school. I would be interested in starting a rice paddy with some of our students. I'm sure this would be a great learning experience for them.
One of my favorite places in LA is the Japanese garden in Van Nuys. Its an incredibly peaceful place. Hard to believe it is a water treatment center. I have gone there several times just to write. The admission is super cheap. It would be a great place for a field trip.
I just want to second the recommendation for the Japanese Garden in Van Nuys. The field trips are well done; the docents show the kids the bonsai garden, the main garden, which includes a Zen rock garden, a replica tea house, and also the water treatment center. Along the way, they explain the significance of different flowers, animals, etc. If possible, you should try to go when the cherry blossoms are blooming. http://thejapanesegarden.com/