Devote your next day to the Mall, where you can check out the museums and monuments that were probably your prime motivation for visiting D.C. in the first place. There's no way you can do it all in one day, so just play favorites and save the rest for next time. Try visiting the monuments in the evening: they remain open long after the museums are closed and are dramatically lighted after dark.
Keep in mind that the National Museum of Natural History is the most visited museum in the country, while the National Air and Space Museum, the National Gallery of Art, and the Museum of American History aren't far behind; plan for crowds almost any time you visit. If you visit the U.S. Holocaust Memorial Museum, plan on spending two to three hours. If you're with kids on the Mall, take a break by riding the carousel.
Cafés and cafeterias within the museums are your best option for lunch. Two excellent picks are the Cascade Café at the National Gallery of Art and the Mitsitam Café at the National Museum of the American Indian, where they serve creative dishes inspired by native cultures. Just north of the Mall, the Newseum features a food court with a menu designed by celebrity chef Wolfgang Puck. If you have more time (and more money to spend), drop by The Source, a ritzy Puck-owned restaurant behind the museum.
If the weather permits—and you’re not already weary—consider the healthy walk from the Washington Monument to the Lincoln Memorial and around the Tidal Basin, where you can see the Jefferson Memorial, the FDR Memorial, and the new Martin Luther King Jr. Memorial. Nearby, nestled north of the Mall’s reflecting pool, is "The Wall," a sobering black granite monolith commemorating the 58,272 Americans who never returned from the Vietnam War—a design that’s "not so much a tombstone or a monument as a grave," in the somber words of writer Michael Ventura.