Painting like Michelangelo - my class STILL talk about it. It was all over the the floor and on their hair and clothes, but it was super fun! This is also great for students struggling with fine motor hand writing. It builds strength in pincer grasp.
How to keep a child busy for hours in the summer: DIY: Giant Ice Cube Awesomeness - filled with plastic animals, dinos, and such. And I'm sure you could put in food dye before its frozen to color it like the ice block in the photo.