This awesome New Zealand hotel built out of a hillside, and a defunct train and plane, has three sleeping options:
"Sleeping inside a 1950’s Bristol Freighter Plane refurbished into 2 beautiful motel rooms.
"Sleeping inside a 1950’s Rail Carriage 3 room motel unit, which sleeps six.
"Sleeping like a Hobbit–underground with a circular window."
That is from Boing Boing, photo included, I am not sleeping there but I would select the Hobbit option. Here is the original link with geographic information as well. Here is a site Unusual Hotels of the World.