Inside the Hancock Observatory, Chicago
Don't be fooled by this picture. Yes, the observation deck is enclosed, but it is still very high up above the city. On the one hand, this offers unique views of the city and some great opportunities for photography; on the other hand, and especially if you are terrified of heights like me, it can be quite a challenge.
Should you be suffering from acrophobia, let me tell you it can be done, but be prepared, especially if you want to take pictures. Since there are windows all around, you will have to lean in with your camera and touch the glass with your lens to avoid reflections. That can be tough to do, but is also so rewarding and worth every drop of sweat.