The Habitable Planet site has some excellent interactive demos for learning about the environment. According to the site:

The Habitable Planet is a multimedia course for high school teachers and adult learners interested in studying environmental science. The Web site provides access to course content and activities developed by leading scientists and researchers in the field.

I was particularly impressed with their interactive Carbon Lab. Provides a nice big-picture sense of how carbon is being transferred from fossil-fuel resevoirs into atmospheric, forest, soil, ocean surface, and deep ocean resevoirs. My 9 year old son was interested in the simulation and seemed to grasp what was going on - he had a puzzled look on his face for awhile as he tried to absorb the meaning of the carbon transfers and whether it was a "good" thing or a "bad" thing.