I checked out a Bartholowmew and the Oobleck book from the library.
After I read my kids the story we made oobleck by mixing 1 cup of cornstarch with 1/2 cup of water and mixing in green food coloring. You can add more cornstarch to the mixture to make it thicker or more water to make it thinner. I gave them different sized containers, plastic frogs and army men to play with. It looks really messy but it washes up really quick and easy.
For the snack I mixed green food coloring with vanilla pudding to make an oobleck dessert. My kids loved this activity.