I love this idea from Kids’ Nature Spot! Not only is it simple and kid-friendly, but she uses homemade flour dough so it’s inexpensive and you don’t need any special materials. This would be a great way to keep pretty rocks the kids collect, too!
If your kids are as crazy about color as mine, you could also tint the water with some food coloring before mixing the dough to make colored dough. I know we grownups tend to like the natural look but most kids I know think the more color the better!