(Sample link) NASA for middle-school students http://www.nasa.gov/audience/forstudents/5-8/features/index.html
(Sample link) National Geographic for Kids http://www.nationalgeographic.com/kids/
(Sample link) Fun activities - by girls....for girls. http://www.missoandfriends.com/
Find out how much you weigh on other planets. Gravity in space
Learn the basics of gravity