What a great way to take summer brain training outside into the sunshine– get that vitamin D along with thinking!
You can use cut out shapes or draw chalk shapes. Graduate from shapes to numbers, from a 3×3 grid to a 4×4 grid. Lots of possibilities here.
And if you need a refresher on how to play- here’s the rules for this shape based sudoku: http://math.about.com/od/recreationalmath/ss/KidsSudoku.htm