On a hill overlooking the Potomac River valley (but not the river), Woodlawn Plantation was a 2,000-acre section of Mount Vernon, of which some 130 acres remain. George Washington gave it as a wedding gift to his adopted daughter (and Martha's granddaughter), beautiful Nelly Parke Custis, and her husband (and Washington's nephew), Maj. Lawrence Lewis, when they married in 1799. Three years later, they moved into the Georgian-style brick mansion designed by William Thornton, first architect of the U.S. Capitol, and furnished it primarily with pieces from Mount Vernon (everything you see dates to before 1840, with about 30% from the Lewis' time).
On the other side of the parking lot, you leap 150 years ahead architecturally to Frank Lloyd Wright's modernistic Pope-Leighey House, designed in 1940 for the Loren Pope family of Falls Church. Built of cypress, brick, and glass, the flat-roofed house was created as a prototype of well-designed space for middle-income people.
Give yourself 1 1/2 hours to digest both houses. Normally given on the hour and half-hour, tours are limited at Woodlawn during March, when it hosts one of the nation's largest annual needlework exhibits.