I love exploring old, abandoned buildings. One of the largest in the world is the Buckner Building, in Whittier, Alaska. Then of course, we have the entire city of Pripyat.
For the person who went to this sanatorium, go to Tropical Islands sometime, in Brandenburg. My dad took me there and we had fun, but I also got to explore around it. Right next to this, there is a huge, abandoned Soviet military base. There are also the remains of what appeared to be an old, abandoned, unlisted concentration camp, later used by the Soviets.