Follow the river north from New York City to find stunning scenery and thriving cultural scenes.Rooftops in Troy, New York.Credit: DenisTangneyJr/Getty ImagesLess than two hours from New York City, the Hudson Valley has long been an easy escape for city dwellers—and you don’t even need a car, with Metro-North and Amtrak trains tracing the river north. Across its nearly 150-mile stretch, the region is studded with small towns that mix history, cu…