My uncle, when young and in school, had a friend who worked as a security guard at what is now again the Biltmore in Miami. Between its times as a hotel it served as a VA hospital, though when his friend worked there it had ceased to even be a hospital. So his job was to make sure no one broke in to steal the stuff left over from its hospital days, and my uncle would often accompany him on his rounds. The building was supposed to be haunted, so one night they took several anatomical models and strung them up inside a corridor so that when the door to the corridor opened, the dummies would fall on the person who came through the door. My uncle and his friends invited a female friend to join them on the rounds that night and urged her to go through the door. She went through the door, the dummies fell, and she fainted dead away. It frightened my uncle and his friend so badly they swore to never pull that kind of prank again.
My uncle has lots of cool stories about the Biltmore.
Personally, one of my favorite vacated spaces is
Fort Casey. It's an old fort on Whidbey Island in Washington, and it's so interesting. There are some rooms of the fort you cannot get to because of the water level, but as the water ebbs and flows you can get into different parts of the fort. It's so eerie. Most of the fort has no lighting, so it has to be explored by flashlight. Similarly,
Fort Stevens in Oregon is a very cool place to explore.