The Greenbrier! My husband and I went there for our 25th anniversary! The place is magnificent. I especially recommend the spa. The bunker was mainly what we came for--but you can only see a few preserved rooms (a bunk room, a cafeteria, the infirmary); the rest has been bought by a railroad company, which now uses the majority of the space for data storage.
When we were there 7 years ago, a van drove us right through the massive steel doors underneath the main resort; however, I've heard that they don't do that anymore.
So if I were going back today, I wouldn't put my hopes in the bunker. It's a vast old resort that is amazing to explore. The spa, as I've said is out of this world ( I loved the Scotch Spray). The food is excellent--and now apparently they've transformed what was an abandoned nightclub from the 1960s into a classy casino.
So I definitely recommend it--but sadly, no longer for the bunker.