Some good books for this subject are: Ecomazes by Roxie Munro, The Whole Green World by Johnston. We made a lowercase e to look like the earth but coloring it green and blue (http://www.notimeforflashcards.com/category/lowercase-e). We also made an earth out of a styrofoam ball. And made a collage of all the beauties of the earth we are thankful for.
More earth day ideas can be found here: http://www.thecraftycrow.net/2010/04/earth-day-crafts-treats-roundup.html