Mystery Science Lessons
https://mysteryscience.com/school-closure-planning
Activities for Grades K-5. Each lesson begins with a Mystery. Kids' are hooked with a quick video. Every lesson concludes with simple hands-on activities designed to use supplies you already have (or are easy to get).
National Geographic for Kids
https://kids.nationalgeographic.com
Games, videos, articles about animals and other science topics around the world.
National Inventors Hall of Fame
https://www.invent.org/at-home-learning-resources
Lots of STEM ideas that can promote creativity and innovation. Here's a direct link to a pdf filled with ideas: Top 10 STEM Activity Guide. This pdf is also on the invent.org site (listed aboove) and it gives ideas for hands-on, indoor and outdoor fun projects for kids.
Club SciKidz
http://www.clubscikidzmd.com/blog/
Daily Science Learning with Club SciKidz
NASA Kids Club and STEM activities
https://www.nasa.gov/kidsclub/index.html
https://www.nasa.gov/stem-at-home-for-students-k-4.html
This is a site for kids to learn about NASA.
Exploratorium
https://www.exploratorium.edu/learn
The Exploratorium is an ongoing exploration of science, art, and human perception. Explore the collection of online experiences and education resources that support online learning.
VIRTUAL Field Trips (also listed on the Social Studies page)
https://artsandculture.withgoogle.com/en-us/national-parks-service/parks
Volcanoes, caves, canyons and more! Explore some scenic national parks!
http://bit.ly/2vDdXhk
Virtually visit zoos, museums and beyond.
https://www.virtualfieldtrips.org/
Educational videos for social studies, geography, life science, and ancient civilization curricula.