The mansion's 27 acres of enchanting formal gardens are straight out of a storybook. The springtime blooms – including heaps of cherry blossoms – are stunning. The mansion itself is worth a walk-through to see exquisite Byzantine and pre-Columbian art (including El Greco's The Visitation) and the fascinating library of rare books that date as far back as 1491. From November to mid-March the gardens are free (and they close at 5pm). Enter them at R and 31st Sts NW.
In 1944 diplomatic meetings took place here that laid the groundwork for the UN. The trustees of Harvard University operate the house, so Harvard students, faculty and staff get in free.