I think the National Mall is one of the most beautiful places in the world. It's a monument to free speech, democratic government, community, intellectual pursuit, and citizens' sacrifice for their country and the world. Even though obtaining an understanding of the politics and practicalities that go into managing the Mall would probably be like touring a sausage factory, I love it.
The Mall wasn't looking very picturesque when I visited, but the open sections were still pleasant to walk through and around. All the museums and memorials, as well as the Smithsonian Metro stop, are still accessible from the Mall. A complete walk around the Mall, even if you don't stop to look at the monuments or in the museums, is a reasonably lengthy trip. Wear comfortable shoes and take water, food (unless you don't mind paying for the overpriced fare the Mall concessionaires sell) and weather-appropriate clothing and accessories. Screen on the Green, the Smithsonian's Folklife Festival, and other events take place on the Mall, so check your travel book or DC event listings to see if anything will be going on while you're in town.
Don't drive into downtown DC if you can avoid it. Parking is limited and traffic is terrible. Take public transportation or a taxi. Or walk - Washington is a pedestrian-friendly town. If you're staying within walking distance of the Mall you're like to catch a lot of other historic and cultural sites just getting where you want to go.
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