As an elementary school OT, to practice coloring inside the lines (when working on fine motor skills, keeping the wrist still and just moving the fingers, visual attention, distal control, etc) we make "robot space caterpillars". We use small grid paper and practice outlining the shape before coloring it in. We usually do about 5-6 boxes at a time.
It's definitely more fun to make robot space caterpillars than to color in boxes. :)