I think there are so many materials out there for young children that are great for hands on activities. I think one of my favorite STEM activities for this age just involves recyclables. You collect toilet paper rolls, wrapping paper rolls and paper towel rolls. (Cut up pieces of cove molding also work great but not needed). Then with your room objects (EX. Block center, desks, chairs, etc.) you create a marble course with the recyclables. You can use all different types of balls (ping bong, bouncy balls, pom poms, etc.) for the children to experiment with what kind works best. Make sure that none are small enough to fit into their mouths.