3. Andrew Carnegie’s Mansion, 2 E 91st St
Traveling north on 5th Avenue, you’ll find the former home of steel magnate Andrew Carnegie. This Upper East Side mansion was designed by the firm Babb, Cook & Willard and built from 1899 to 1902. It boasts a whopping 64 rooms and a stately English Georgian style. You can wander those rooms today as you explore the exhibits of the Cooper-Hewitt, Smithsonian Design Museum which now occupies the home.