The chemistry channel of about.com has instructions on how to grow crystals from epsom salt. All you need is warm water and the salt which you can find in the laundry section of most stores. You can also add food coloring for fun or a sponge to have a base.
Good for ages 3 and up.
Get the instructions here:
Kix cereal creates the crystals in a plastic lid for hanging and demonstrates them in a video.