Baking soda and vinegar.

We have found a lot of crafts on pinterest that Charlie loves and a lot of them have baking soda and vinegar in them.  At Halloween we made witch potion in a cauldron and our latest creation is sidewalk paint.  We spray a bottle of baking soda and cornstarch first and then use a spray bottle for vinegar.  It bubbles up on the sidewalk and entertains Charlie for awhile.