Here's a baker's dozen of links (in absolutely no order) that takes you beyond Earth Day Network. These will connect you to a multitude of resources out there to inspire you for planning your Earth Day classroom (or even home) activities.
The best part... these aren't just limited to Earth Day. It makes for a grand excuse to make everyday Earth Day!!
- Teachervision: Earth Science Resources (Specifically including Earth Day)
- Marine Science Institute: Bring Earth Day to Your Classroom With Educational, Engaging and Fun Activities