Live in a Castle Like Royalty, with Royalty
So what if you didn't just go visit a castle somewhere, or even just stayed in a castle-turned-hotel somewhere, but actually stayed in a castle with the royalty who owned it? Now that would be an experience. It's basically a different spin on the "bed and breakfast" idea, where you get to stay in a real castle with servants and butlers, walk in the gardens, eat in the main hall, admire the ancient portraits of royals gone before, and then on top you get to have tea and visit with the lord, knight, or princess that happens to live there too.
The only part that bothers me about all this is the idea that most of these people are opening their homes not because they want to share their history, or because they enjoy mingling with "commoners," (although maybe they do) but mostly because it just costs so much to keep those massive castles running. Maybe that makes the experience that much more genuine though -- amidst the luxury and royal treatment an underlying air of haughtiness from the nobles would definitely fit in with history!
(Ahem, I'm sure they're all very nice, really).