If your kids are getting tired of hopscotch, try using that sidewalk chalk to make a sundial. My kids were blown away that it really worked. They even used it to tell what time to come in for dinner!
Crayola has great, printable instructions for making your own human sundial. All you need is chalk. And a human. And the sun. And if this project really sparks an interest in your kids, the North American Sundial Society has an incredible website with extensive instructions, facts, history, and links to sundial artists' creations.