One of the best ways to help kids care about the earth is to start them off by caring for a microcosm of it. This is why we love terrariums so much. (That, plus the gnomes.)
Creating and maintaining a terrarium is an opportunity for countless science lessons, from building a healthy soil ecology to learning about the various energy (light and water) requirements among different plant species. What happens if you overwater? Don't provide enough drainage or ventilation? Plant aggressive species among non-aggressive?
National Geographic Kids has a great (and by "great" we mean "easy") tutorial for building a kid-friendly terrarium for kids. Their directions include a list of plants that adapt well to terrarium life. All you need to get started is a glass jar (we like this traditional candy jar), some small stones, horticultural charcoal, potting soil, and of course, plants.