Homemade bath crayons - Buy some glycerin soap at a craft store. Melt it in the microwave, add food coloring, and pour into a mold. I used an ice cube tray. Takes about an hour to cool completely, then pop out of the mold and you have crayons!
All you need is a bottle of shaving cream and a box of corn starch. This was enough for two tubs of this wonderful, fluffy stuff! We poured about half the box of corn starch in each tub and then just squirted the shaving cream into them.
This is an awesome website for kids' science experiments. How to make an egg shell disappear without touching it. Lots of fun science experiments on this website, along with explanations for the kiddos.