The Presbyterian Church (USA) has a curriculum for youth titled "Stewardship: Choices". It has 8 topics, one being on stewardship of creation. There is a complete lesson, plus cards for discussion starters. My junior high kids love this!
You could do recycling either with the kids or the whole church.
Or, contact your Cooperative Extension Agent and learn about renewable resources, water quality, etc.
There is a book (not religious) titled "What On Earth to do With Kids?", by Robyn Freedman Spizman, Good Apple Inc, 1991, ISBN: 086653623X. It has all kinds of earthy lessons for elementary kids.