Here is a neat craft/experiment where not only will your child end up with a fun item but they might just learn something in the process, like the fact that some things don't mix! All you need is a bottle, vegetable oil, water and some food color.
First put in the vegetable oil about half full in your bottle. Next mix food coloring with the water in a separate cup. Carefully add the colored water to your bottle. A funnel is very helpful for this and you can usually pick up a set of funnels at the dollar store.
Now for the science! Once the lid it tightly on, have your child gently lay the bottle on it's side. What happens? Slowly move it back and forth to see how the oil and water stay separated. Next shake the bottle up hard! What happens? Let the bottle sit to see how the oil and water separate again. Now just let the kids have fun watching the "lava" flow!