We grew our own crystals using bicarbonate of soda (NaHCO3)!! These Usborne Science books teach children about science in a fun way and includes simple science experiments you can carry out in your own kitchen. We have the Chemistry, Physics and Biology books which we bought quite cheap at The Book People!
1) Fill 2 jars with hot water and stir about 6tsp of bicarb into each jar. Keep adding more until no more will dissolve.
2) Put the jars in a warm place they will not get moved with a plate between them.
3) Cut a piece of wool as long as your arm. Tie a paperclip to each end of it, and place one in each jar.
4) Leave the jars for at least a week. Crystals will start to grow along the wool and hang over the plate.