Webrangers is a fun interactive website, which is actually the National Park Service's online Junior Ranger program for kids of all ages. You will find lots of games and activities plus the opportunity to earn rewards. There are activities concerning nature, history, science, animals, famous people, and the national parks. The skill levels are easy, medium and hard so all ages can find things that are the right level for them. Kids can customize their own online ranger stations as well.
The best part is that it is FREE to join and play! It truly is a great resource.