What to Pack
Washington D.C. is a fairly laid-back city. If you plan on visiting the tourist sites, comfort should be your priority: jeans (or shorts in the summer) and sneakers or comfortable walking shoes. If you're visiting a lot of museums and plan to shop, bring an empty backpack, or you'll be carrying around shopping bags loaded with souvenirs all day. Be prepared to check your backpack at the door of every attraction and to have your belongings screened by security.
If you know you will be attending an arena event, an outdoor concert, or parade, you might want to pack binoculars. Also, don't forget your camera and/or video camera.
Spring and late summer are often rainy, so pack a collapsible umbrella or a rain poncho and put it in that knapsack. In the summer, pack a bathing suit. Many of the hotels have pools, and you'll want to cool off.
If you are planning on going to the theater, the Kennedy Center, or some of the better restaurants, plan to dress up a bit. But even the most stylish of places accept “business casual.”