First, I have to tell you that it's an enormous luxury to have so much space that there are rooms you don't know what to do with! Sounds to me like the room is already suggesting what it would like to be...mini fridge says bar to me, which can evolve into a library, loungey, den. Paint the walls dark, use lamps instead of overheads to create a subtle vibe and go with heavy wood and leather pieces. I know this sounds expensive, but I've put together spaces exactly like this sourcing only from thrift stores and hotel liquidators. (the good news is, this slightly masculine, pure classic look is best achieved using pieces with character- i.e. slightly scratched, worn...affordable!) I can already picture you in there- glass of wine, a good book...
Happy hunting!
KC