Strasbourg

Strasbourg needs 2 days: Notre-Dame Cathedral (with its single spire, the tallest built entirely in the Middle Ages) and the Petite France district, with half-timbered houses over the canals of the Ill River, on one; the Rohan Palace and the European Quarter -home to the European Parliament- on the other. April-September has the best weather, though December brings France's oldest Christmas market, running since 1570. It's cheaper than Paris. It's the de facto seat of the European Union alongside Brussels, and its historic center, a complete island surrounded by the Ill River, was the first urban center to be declared a UNESCO World Heritage Site in its entirety.