Earth Day will be celebrated on April 22, 2010. Visit these sites for activities for your classroom.
http://www.mathwire.com/seasonal/earth.html Although primarily a math site, many of the Earth Day activities found here have a science component as well.
http://earthsbirthday.org/earth-activities/ There are items available to purchase here, but if you poke around a bit, you will find many free activities and handouts across grade levels, many in PDF format.
http://www.seventhgeneration.com/give/earth-day-lesson-plan Nice collection of Earth Day lessons, spanning grades 2-12.
http://www.lessonplanspage.com/EarthDay.htm Great site from HotChalk. All grade levels.