United States Capitol: The most magnificent building in Washington, the U.S. Capitol Building is where visitors go to witness politics in action. Inside, members of both houses of Congress debate and create national policy and law, while visitors explore the building's north and south wings and circular centerpiece.
The National Mall: The best way to see most of Washington's monuments and museums is to stroll down the Mall, a greenway crowned by the Capitol and lined with treasure all the way to theLincoln Memorial.
The Vietnam Veterans Memorial: (the wall) One of the most moving war memorials. Is a long black granite wall with the names of more than 58,000 Americans who perished during the Vietnam War emblazoned on its surface.
The Washington Monument: It was the tallest structure in the world. And nowadays, you can ride one of the Monument's glass-encased elevators to the top observation deck to enjoy 360-degree views of the city.
The White House: Home to every U.S. president since John Adams in 1801, the White House is America's most famous homestead.
Source: http://travel.usnews.com/Washington_DC/Things_To_Do/