General Challenge Ideas
School Library Journal has curated a great list of sites that have challenge cards that use Legos, Dash & Dot, Scratch, and Stop Motion! Don't be scared off by the teen label- as they are perfect for your elementary students.
ICCSD Librarian Jackie Biger's list of challenges that she has created and uses at Lemme Elementary.
James Dyson Foundation (yes, the Dyson of Vacuum fame!) has awesome STEM challenge cards available, but they do require prep and material gathering like most science experiments do. Check out the orange engineering challenges.
Grant Wood AEA is a great resource for lesson ideas! Check out their site: Think Make Innovate.
Teacher Gretchen Renshaw has created a blog with great lesson plans.
Loads of ideas on Makerspaces.com