The AAA city map of DC is good, showing locations of most of the museums, monuments, and Metro stops. The map in the guide to the Smithsonian Museums is good for things right along the Mall, but not so good beyond that. An online resource that I've found useful is www.stationmasters.com, which shows exactly where you exit and enter the Metro (though the maps of the surrounding areas are a little out of date). Even after living here for 16 years, it can be easy to get turned around when you emerge from a Metro stop you use very infrequently. And as GoingAway implied, there are a lot of FTers with excellent knowledge of DC who are more than happy to help and offer suggestions.