Rocking, swaying, spinning , swinging & rolling are very important balance activities to stimulate the vestibular (balance) system. Children of all ages, right from birth, need constant daily stimulation of the vestibular system.
DIY Turn PVC pipes into a playground musical instrument. I wish this had the instructions with it, but I might be able to figure it out. This might even work as a service learning project for my daughters school.
Crawling is the first time children use cross patterning movement (opposite arm, opposite leg action), that is necessary for the development of the pathways that connect & send information between the left & right sides of the brain.