I can recall vividly my father introducing me to coloring in sheets as a hobby, and now I am doing the same with my child. Never mind how the world changes, our children nowadays love to color in just as much as we did when we were young. Kids love to show off what they can do by using their paints on the coloring sheets you give them.
Studies have shows that anxiety levels dropped in adults that colored. They did note that simple doodling had no affect on anxiety. The focus on coloring and switching off the brain allows that blocking of anxiety in the moment. Coloring does not have a need for complicated thought processes and like listening to calming music, you are able to get within yourself, isolated from external anxiety, commotion, and distractions.
To test the wits of the toddlers you can give them coloring puzzles. Let them first try to complete the puzzles and then color them once done. Since toddlers hold a perspective of what color an image should have, let them do the color they like. However, as they advance, try to push them to make the coloring resemble the actual looks of the thing represented by the images in the coloring books.