Pinterest can be a great resource for fun, DIY STEM activities. Try this one, or any of the dozens of other boards devoted to STEM fun.
The Ultimate STEM Guide for Kids 239 Cool Sites including fun facts, activities, contests, camps, career advice, and anything else you might be looking for!
iHomeschool Network has any number of activities, including STEM, which are easily done at home.
100 Awesome STEM Resources from Amy at Milk and Cookies Blog
100 Super-Fun STEM Resources for Kids by Colleen at Raising Lifelong Learners
STEM Resources from Technology Education
Science Kids has all kinds of fun science games and activities, great for 3rd-5th graders.
Skills for various industries from DIY.org
Ask Dr. Universe - Q & A's for all of life's most burning STEM questions, asked by kids around the world.
Interactive is a free computer game for learning in all sorts of STEM realms.
Bending light - Learn how to bend light using just water in a clear container.
A2Z Physics for Kids Links to all sorts of fun physics activities and learning opportunities.
Astronomy Projects from DIY.org.
Science Fair Project Ideas from Science Buddies
Biologist Skills from DIY.org
National Ecological Observatory Network has a great Citizen Science Academy that allows any interested person to help collect data on the nation's ecosystems!
100+ Ideas for Nature Study from Cindy at Our Journey Westward
100 Things to do Before, During, or After a Nature Walk from Jimmie at Jimmie's Collage
Technology & Engineering
Code.org - Learn to make apps, or learn the basics of how computers think.
Engineering Go For It! - Links to everything from news to career advice on becoming an engineer
DiscoverE - Fun activities celebrating Engineering. They also have free volunteer kits available to learn about being an engineering mentor.
100 Learning Resources for Kids Who Want to be Game Designers and Programmers from Aadel from These Temporary Tents
100 Engineering Projects for Kids from Marci and The Homeschool Scientist
100 LEGO Learning Printables from Renee at Great Peace Academy
Aviation Careers from Learn How to Become
Fun math activities from Education.com
100 Living Math Activities from Cindy at Our Journey Westward
Free tickets to the Museum of Science, New England Aquarium, Zoo New England and many others are provided to anyone with a Boston Public Library card.
Check out NASA's info for young learners.