Okay, so you might not actually take a ship there, but you can visit Highclere Castle — the "real" Downton Abbey — by booking the Oxford & Highclere Castle Extension with your Cities of Light river cruise. The three-night extension gives you a chance to explore the grounds at Highclere — including the main hall, dining room, library, and sitting rooms — as well as Oxford University. All that takes place before the real cruise sets sail on a 12-day trip from Prague to Paris, with stops in Luxembourg, Trier, Cochem, Heidelberg, and Nuremberg along the way. [via]