• Nature Challenge for Kids - This David Suzuki Foundation website starts out with ten simple ways you can protect nature, followed by four challenge activities that offer first-hand experience with the natural world. 
• Kids Planet – Species fact sheets, “wild games”, web of life, how YOU can help defend the environment, even a Wildlife Adoption Center. 
• The Big Blue Bus – A fun way to learn about protecting the world’s water sources.  Become a Water Wizard and play games and activities while learning about “all things watery.” 
• Web Rangers – Presented by the National Park Service. Activities are presented for three age groups (6+, 10+, 13+) in categories Adventure, Exploration and Discovery. Challenges, puzzles, games and activities about nature and stewardship. 

Thanks to Mrs. McVey's class for the following suggestions of other great links!

Water Conservation for Kids
Recycle!
Science Project Ideas
Endangered Animal information
Energy Kids
Kids & Conservation 

[Thank you to Earth Easy for great link recommendations]