One of my favorite things that was shown to me during my elementary age years was an optical illusion. It was the very famous Rubin Vase (pictured below) which you can see in either two ways - as a vase or as two faces that are facing each other.
Built in tree book shelf - Inspiration Only but this is gorgeous. I so want this in my next home. There is also another tree on my project board I want to put in the same room as this. This would be great for a kid& room.