The federal government in the USA exists out of three branches: The legislative - the Capitol, The executive - the White House, The judicial - the Supreme Court, and they are all located around the National Mall, the only mall I like.
Legislative
And the time is: 1787 to now
And we are in: The Capitol
Ten years after the Declaration of Independence the Congress had to tackle the issue of representation difference between small and large States during the Philadelphia Convention of 1787. The Connecticut delegation proposed a bicameral legislature, also known as the Great Compromise. A House of Representatives (now with 435 members) representing States according their population, was established. Today, the Speaker of the House, Nancy Pelosi, is the leading officer and the 3rd in line for the presidency. The House comes together in the South wing of the Capitol. Each 10 years a census will count the population of each State, with the next one coming up in 2010.
The Senate (now with 100 members) two Senators from each State was installed. Today, the Vice President Joe Biden is the president of the Senate. The Senate comes together in the North wing of the Capitol. Together they form the Congress. The Constitution giving both the responsibility for declaring war, maintain an armed force, assess tax, borrow money, mint currency, regulate commerce and make all laws necessary for the operation of the government.
What do I do: I tour the Capitol and the Library of Congress close to it.Judicial
And the time is: 1801 to now
And we are in: The Supreme Court
John Marshall declared the Court to be the supreme arbiter of the Constitution and gave shape to the constitutional balance of power between the federal government and the States. And so the Supreme Court became the highest judicial body and the leaders of the federal judiciary. 
Today existing out of a Chief Justice, John Roberts and eight Associate Justices, nominated by the President with the latest, Sonia Sotomayor, appointed by Obama only last August. The Court looks at some hundred cases a year and comes together only from October to April for Oral arguments. 
What do I do: I get a lecture and a tour of the Supreme Court.Executive
And the time is: 1789 to now
And we are in front of: The White house
From George Washington to Barak Obama, the president of the USA is the head of state and the head of government and the leader of the executive brand of the government. From the second president John Adams, his home is the White House. The President with his presidential veto, has legislative power, with his power to nominate federal judges for the supreme court, he has judicial influence. Although declaring war is a Congress power, the president is the commander in chief of the USA armed forces. And most of all, being the chief of the executive branch, he takes cares that the laws be faithfully executed. And after the most stupid president ever, its a relieve to have Obama, who is eager to make the world a better place for everybody (nothing wrong with Dutch socialism). With who am I hanging: My friend M commutes from California to Washington DC on a weekly basis and gave me the key of his apartment in Alexandria when we met in New York. And thats not bad.
What else: We see the terra cotta warriors in the National Geographic museum and the Stars Spangled Banner from 1813 (still with only 15 stars), Lincolns hat, the red shoes of Dorothy (the wizard of Oz) and Kermit the frog in the American history museum. I stroll through the Portrait Gallery and the American Art museum. And enjoy for hours my favorite, Anne Truitt in the Hirshhorn museum.
Enough of all the museums and indoor stuff, and when the weather clears up, we hike (well it became more a scramble with all the snow and ice on the trail) the Old Rag loop trail in Shenandoah national park. 
Dag,
Iris (Alexandria, 25706 miles)
