Here's a fairly simple activity for your preschoolers and Kindergartners... Make and play with play dough!
It was actually really easy to make play dough... 15 minutes from start to finish. The recipe I used was the first this kiddinaround.com post but I only made 1/3 of the recipe and it still made a lot.
Remember that all month you can add your link to Kindergarten MaTh (and next month is a 1st grade focus). Enjoy your summer!