We gathered a jug of salt water from the ocean 2 years ago on a trip to Tybee Island near Savannah, GA.  I had in mind when I collected it to do an experiment to show the kids how to separate the salt from the water, how crystals formed, how to research stuff, etc.  I finally got around to it last weekend.  We did several experiments which have had mixed success.  First, we looked at various things under the microscope including epsom salts, table salt, and sugar.  In our first experiment, we suspended a string in a cup full of ocean water.  No crystals formed on their own so we are waiting on evaporation to expose the salt.  We also mixed epsom salts and water to form crystals.  This was sort of cool but not as dramatic as I had hoped.

Next, we supersaturated water with table salt.  Crystals have definitely formed on a suspended string.  Crystals have also formed on the side of the jar in which we are doing our experiment.  We put a bunch of salt in that jar so the effect is pretty dramatic.  This was closer to the effect for which I was hoping.

Both of these experiments were pretty slow to show much effect for kids though.  Being anxious for my kids to ohh and ahh at my scientific knowledge and ability, I found a recipe for making crystals from salt, liquid bluing, and ammonia. 

As a base, we tried to use a piece of cardboard rolled up in a tube.  My expectation was that the cardboard would wick up the water and make a little crystal tree.  For some reason, the cardboard did not wick any moisture.

I guess it was too dense (or maybe I was?) so the crystal tree turned into more of a crystal shrub.  Still, the effect started within a day or so.

Anyhow, here are some additional methods to make crystals.  The kids were in to it for awhile and had a good time checking on the progress.  I’d recommend it for your own edification as well!
http://britton.disted.camosun.bc.ca/snow/boraxsnowflake.html

http://www.squidoo.com/groweasycrystals

http://silken-didyouknow.blogspot.com/search?q=crystal

http://www.science-projects-resources.com/crystal-science-projects.html