There is everything from Trees to Water. This is a good collection of lesson plans about Weather. There are activities for both primary and intermediate students. The Environment Theme Learn about the rainforest and environment and get valuable lesson plan ideas here. You Believe It for grades K-3.
Children click on items in the picture made from wood products.
There is also a Knowledge Trivia quiz for older students and adults. This is an interesting site to check out if you are teaching an Environment or Tree theme. This site has lots of information and tips on keeping our earth green. The Kids Page has an interactive quiz and fun links to useful stuff and an interactive puzzle game. This site has Earth Day activities from Hawaii.