Barcelona

Barcelona blends a Mediterranean city, modernist architecture and urban beach into one walkable itinerary. 3 days is enough for the essentials (Sagrada Familia, the historic center and a beach afternoon), though 4-5 days lets you take it slower through the rest of the Gaudí trail. May-June avoids extreme heat and the summer cruise crowds. Prices are mid-range for Western Europe, more affordable than Paris or London, though the tourist center rises in high season. It's one of the few big cities in the world where you can visit a museum, eat tapas and be at the beach the same afternoon, all on foot — and the Sagrada Familia is still under construction more than 140 years after it began.