We don't really have any nicknames for our rooms. We just call them what everyone else calls similar rooms. However, my SIL has a guest room in her house. It became kind of catch-all/storage room (it was a mess). My niece, who was about 3 at the time, called it "the mystery bedroom" because you never knew what you might find in there. My niece is 19 now... and the whole family still refers to it as "the mystery bedroom."