Avocado.org has instructions to grow your own avocado tree. Kids will love seeing the pit grow roots and leaves. They even have a troubleshooting guide in case things go awry.
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.