A couple of weeks ago, I made a magnificent trade on Swaptree and received My First Science Book, a book of simple experiments that kids can do at home. The pumpkinhead perused the book cover to cover, and decided on the spearmint called "Volcano in a Jar", which is basically a study in how heat rises.
This was fun enough, but involved only water and one shade of food coloring. It left the pumpkinhead wanting more. So after changing out of her school uniform, she got to use the whole set of food coloring and many, many more jars and cups for even more scientific fun while I made dinner.
Her further experiments involved lots of blue and brown water, lots of mixing of said water, and some baking soda and vinegar for another type of volcano.
She was pretty happy with her spearmint. I worked around her lab area and got dinner ready, then we shoved everything into the dishwasher after we ate. Yay, science!