Practice makes perfect, or whatever

Little Man has to be bribed a bit to practice his piano. The trouble is he has an excellent memory, and can play the songs very quickly. That doesn't mean he keeps the right posture, fingers over the correct keys, etc., he just knows which keys to press. He was promised a special small surprise for each week in which he practices ever day. I have no idea what the surprise could be. Maybe something to eat he loves? Something we can bake together? Hmmmm....