These websites have activities for many subjects and links to other cool sites!
http://www.kids.gov/k_5/k_5_fun_activities.shtml - Wow! This Kids.gov link has an incredible amount of links to other amazing websites about a variety of topics! A few of the many options include art sites, outer space activities, beaches, government, codes (cryptograms), defense intelligence, energy, the environment, farming/fishing, nutrition, money, Garfield comics, animals, PBS, and more! There are also games and activities for little kids here (such as Clifford and Sesame Street). Check it out!
http://www.funbrain.com/ - Funbrain has math and reading games and interactive activities.
http://iptv.pbslearningmedia.org/ - PBS Learning Media has videos and interactive media for all grade levels and subjects.
http://www.scholastic.com/teachers/student-activities - This Scholastic Student site has activities, videos, simulations, online field trips, games, and more for all grade levels and subjects.
http://www.hoagiesgifted.org/links.htm - Hoagie's Kids and Teen Links includes links to brain teasers, optical illusions, theater and the arts, books and authors, cartoons, games, geography, math, language, science and social studies, virtual reality, volunteering opportunities, and sites for young kids.