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.
DIY Nerf darts — when the original set gets lost (and you know they will get lost!), you'll be happy to know that you can make your own Nerf darts! Now it is known how to get the holes in the Nerf darts for legs on Proto X SLT quadcopter.