There are many sites to visit in the nation's capital (see attached maps). You've probably visited (or planned on visiting) the sites on the Mall like the National Air and Space Museum, the Washington Monument, the Lincoln Memorial, and the National Museum of American History. These sites are all worth a visit.

If you've already checked those out or are looking for something different, here are few alternatives:

National Museum of Women in the Arts
This museum, devoted to recognizing women's achievements in the arts, contains works from the sixteenth century to the present but is especially well known for its modern art (think Georgia O'Keefe).
Location: 1250 New York Avenue, N.W. (very close to the hotel)
Hours: Open Monday through Saturday from 10:00 a.m. to 5:00 p.m.; Sunday from noon to 5:00 p.m.
Phone: 202-783-5000
Website: http://www.nmwa.org/

Decatur House
The guided tours of this National Trust property are fabulous. The home has been restored to display two periods--the lower floor interprets the home as it would have looked in the time of 19^th century naval hero Stephen Decatur and the upper floor has been preserved to reflect the lifestyle of the wealthy 20^th century patron who donated the home to the National Trust.
Location: 1600 H Street, N.W. (right by the White House)

Hours: Tuesday to Friday from 10 a.m. to 3 p.m. (last tour 2:30 p.m.); Saturday & Sunday from Noon to 4 p.m. (last tour 3:30 p.m.)
Phone: 202-842-0920
Website: http://www.decaturhouse.org/

Renwick Gallery
The Renwick Gallery is an oft-overlooked member of the Smithsonian. Housed in a mansion originally designed to house the Corcoran art collection, it now displays a unique collection of American crafts from the 19^th century to the present--the clock on the second floor is definitely worth a close look!
Location: 17^th Street and Pennsylvania Avenue, N.W. (right by the White House)
Hours: Daily from 10 a.m. to 5:30 p.m.
Phone: 202-357-2700
Website: http://nmaa-ryder.si.edu/collections/renwick/main.html

Hillwood Museum:
Just re-opened after an extensive restoration, Hillwood is the former home of Post cereal heir Marjorie Merriweather Post. It features the most comprehensive collection of imperial Russian fine and decorative arts outside Russia and an extensive collection of eighteenth-century French works of art.
Location: 4155 Linnean Avenue, N.W. (you'll need to drive or take a taxi)
Hours: Tours are available by reservation only--you must call ahead to reserve a space (before leaving home).
Phone: 1/877-HILLWOOD or 202/686-5807
Website: http://www.hillwoodmuseum.org

The first time I came to DC and was told to take the Metro, I was (and I am not ashamed to admit it) scared.  The only undergrounds that I had ridden recently were the NYC Subway and Philly's Septa (a.k.a. septic) system.  Quite logically, I was not looking forward to another underground public transportation experience.  I was, however, to be pleasantly surprised.

The DC Metro is organized, clean, climate-controlled, and surprisingly easy to navigate.  While it can still be very crowded, especially in the most popular tourist spots, the stations are large and the signs plentiful.