It's a shock, a very pleasant one, to find such a little-known jewel on this otherwise thoroughly modern block. This rare survivor from the early American republic was built as a carriage house for Abigail Adams Smith, daughter of President John Adams, and her husband, William Stephens Smith, in 1799. It's been painstakingly restored by the Colonial Dames of America to its early 19th-century condition, when the house served as the Mount Vernon Hotel -- a country hotel for bucolic overnights away from the city, if you can believe it. On a guided tour you can explore nine period rooms, outfitted in authentic Federal style, as well as the grounds, planted as a late 18th-century garden would be. An orientation center offers a scale model of the building as it looked in 1799 and screens a video on New York City in the early 19th century. Special events take place throughout the year.