Teach Engineering: Curriculum for K-12 Teachers has lessons, units, and activity ideas across the science disciplines.
Most of the lessons are designed for upper grades, but there are some primary level science gems. I've attached links to some of the K-2 lessons that I found.
Hands On Activity: Touch and Discover
Measuring with blocks:
Hands on Activity: Do Different Colors Absorb Heat Better?
Do Different Colors Absorb Heat Better?
Hands On Activity: Invent a BackScratch from Everyday Materials
Invent a BackScratch from Everyday Materials
Hands on Activity: Which Roof is Tops?
I've also been collecting engineering lesson ideas on my Pinterest Boards. You need to have a Pinterest Account to view the boards. It is free. The following Boards have many ideas to get started with Engineering and STEM activities: