LEGO Gravity Rollers: A Fun Contraption That Propels Itself! Fantastic STEM project for kids.
Physics play- Train Chain Reactions with Dominoes! STEM for kids! via @karyntripp
The easiest and best slime recipe out there - this 3-ingredient fluffy slime doesn't use borax or liquid starch and results in a fluffy, squishy stretchy slime that you just have to play with to believe!