You asked what you should track:
Beyond the basic milestones, some things I am sorry I didn't record or document:
"Wow" behaviors - like my kiddo reciting Living Books software stories when he was two or three, complete with sound effects. "Harry opened the door. Screeeeeeeee. The door squeaked." The stories were 15-20 pages long, and he would play them over and over until he had them completely memorized.
Musical accomplishments - songs he picked out on the piano when he was three. He would play for hours. They showed complexity and nuance, but we never recorded any of them.
Extreme interests - he carried a pocket-sized encyclopedia of dinosaurs with him everywhere when he was three or four. It was about 150 pages, and he knew the details from cover to cover despite not being able to read a word. He badgered all of us into reading the same page until he knew the material.
