I used to enjoy growing crystals when I was a kid. It's a good indoor activity for the winter.

Plain white sugar is also interesting. Alum (used to make pickles, can often be found in the herb and spice aisle at the store) makes neat crystals.

If your kids are old enough to be trusted not to eat the experiments, copper sulfate (used to kill roots that are growing into sewer lines) makes beautiful crystals. It can be found at Home Depot-type stores.

Edit: I see this author has a link to another article on growing copper sulfate crystals down at the bottom of this article. Recommended!