The Hudson Valley is vast and varied. With hundreds of miles of sandstone and granite cliffs, cattail-lined riverbanks, former factory towns, orchards, farmland and forests, the scale of its geography and the scope of its history are daunting. To spend a weekend dropping into its musty bookstores and sizable art institutions or idling between hilltop castles, divey small-town bars and doily B&Bs is like skipping a stone into a river: you bounce along, but barely break the surface. From New York City, it’s a one-hour train trip to Peekskill, at the doorstep of the mid-Hudson Valley, but the region can be fully explored only on the kind of road trip that skirts one side of the river and winds down the other, hopscotching between historic estates and detouring for farm stands, roadside diners and seductive swimming holes.