Rapaire has good list. Smithsonian museums are best accessed from the Smithsonian Station on the Orange and Blue Metro lines, though. Unfortunately, the U.S. history museum is closed for renovations.

Another interesting museum: the new National Museum of the American Indian, on the Mall. It also has the best museum cafeteria food in DC.

For music, Capitol Area Bluegrass and OT Music Association holds jams on 2nd and 4th Sunday of the month in Arlington -- across the river from DC but the jams are in a park that's not easily accessible by public transit. Almost exclusively bluegrass the last time I went. http://www.caboma.org/

The Historical Society of Washngton DC published a Washington Beyond the Monuments map a few years ago. PM a mailing address if you think you will have time for some excursions to lesser-known sites.

And I agree with everyone else: Metro and walk. Avoid the in-town driving.