Websites with multiple activities:

Smithsonian: Fun Stuff for Kids

Science Kids

Sheppard Software

Space Place (NASA)

Websites to learn something new:

Scholastic: Study Jams

PBS (Public Broadcasting Station): interactive activities

Science Buddies: Careers in Science

Monterey Bay Aquarium: online courses

Videos:

BrainPOP: videos

Weird But True: YouTube videos

At-home activities:

Science! KIDS short videos

California Academy of Sciences: @ Home activities

The Tech Interactive: at Home activities