When I picked up some Liquid Bluing at the grocery to whiten my laundry a bit, I was surprised to find a recipe for making salt crystals on the bottle. We started with Salt, cinder rock, water, and of course Mrs. Stewarts Liquid bluing (also great for keeping your white pet nice and white.)
These are the progression of the salt crystals. The salt grew just as well all over the sides of the bowl as on the rocks. It was a fun experiment, the crystals grew continually for about the month that it inhabited a spot on the kitchen counter.
I recommend this project, it's simple once it's started and fun for the kids to watch grow.