Our intrepid four-year old has been collecting paper towel tubes, refrigerator soda boxes, and magnifying glasses. What to do?
Try making a primitive no-film camera demonstrator with a lens-on-a-tube! You can see really bright, colorful images of the TV screen, stuff in the house, and all sorts of stuff outside.
This is the second optics "Learn it Yourself" project, following the pinhole camera in a teabox: