London

London is one of those cities that refuses to be reduced to a highlight reel. The free world-class museums alone — British Museum, V&A, Natural History Museum, Tate Modern, National Gallery — would justify a week. Layer in the pubs (not a cliché, an actual culture), the market food scene, the sheer density of history per square mile, and a parks system that makes you forget you're in a city of nine million people, and you have a destination that rewards both first-timers doing the circuit and regulars finding new neighborhoods every visit. The trick is to treat the Thames as your organizing axis: most of what matters is either on it or a short walk from it. The Tube is excellent; use it, but walk more than you think you need to — the between-stops streets are often better than the destinations.