Lawrence Hall of Science
University of California: Lawrence Hall of Science: Howtosmile
A free collection of math and science activities, games, and lessons that teachers can search by topic and grade level.
Other
National Environmental Education Foundation: Environmental Education Week
Find many activities, resources, and even webinars that celebrate environmental education and its coordination with engineering.
Technovation
Curiosity Machine: Challenges
Curiosity Machine provides free, high-interest STEM activities for students. This home page provides links to categories of STEM activities.