If you ever finish an activity early, any and all of these websites are approved places for you to spend some time. You won't even realize that you're learning something!
Fantastic Contraption
National Geographic Games
Games | National Geographic Channel- National Geographic Channel Some really cool games, simulations and activities on this site.
News for your Neurons
Wired Science - News for Your Neurons | Wired.com Cool current events from all corners of the realm of science.