My seven year old loves his giant sized version of Color Sudoku called Colorku. It is a little pricey; he received it as a gift and it gets a lot of use. I love that he can play the game independently, so it keeps him busy while I'm doing other things.
Colorku (affiliate link) is played just like the traditional game of Sudoku but with colored wooden marbles rather than numbers. This version comes with 104 different puzzles to solve, and additional puzzle cards are available to extend the fun as kids start to mast the game.
In the photos you can see that New Kid (age 3) has appropriated the game for his own purposes. (In fact, re-purposing games for younger kids is a common practice in our home!) He likes to first move all the marbles to the plastic frame...
... and then arrange them by color in the wooden frame. I like to think of this as a fine motor skills exercise.