Westchester County: See
In Mount Vernon, be sure to visit Saint Paul's Church, a National Historic Site.
In Tarrytown, you can visit Lyndhurst, "America's most famous Gothic Revival Mansion", and Sunnyside, the home of Washington Irving, author of The Legend of Sleepy Hollow and Rip van Winkle.
Philipsburg Manor is a historic Dutch manor and mill restored to its appearance in the late 17th/early 18th century. You can visit it and Kykuit, the Rockefeller estate when you're in Sleepy Hollow. While you're there, stop by to admire the stained glass windows by Henri Matisse and Marc Chagall at Union Church of Pocantico Hills, and walk through Sleepy Hollow Cemetery, the final resting place of Washington Irving, Andrew Carnegie, Walter Chrysler, and Elizabeth Arden. It also contains the Old Dutch Church and Burying Ground, the churchyard from Washington Irving's The Legend of Sleepy Hollow.
The Hudson River Museum in Yonkers has exhibitions on regional history, Hudson Valley art, and a planetarium.
In Purchase, you can visit the Neuberger Museum of Art, which features modern, contemporary, and African art, and stroll through the Donald M Kendall Sculpture Gardens, which has more than 40 major sculptures amongst 12 outdoor gardens.
Across the entire county is the Taconic State Parkway, where you can drive through beautiful forests during the autumn months.