In Washington, be sure to visit the Smithsonian's National Museum of American History (diagonally across the Mall from the Air and Space Museum) and check out the new, permanent exhibition, "The Price of Freedom: The History of American Wars." For quite a few years the Smithsonian has been criticized for its out-of-date treatment of American military history. This exhibition is its effort - generally successful, in my opinion - to catch up.
If you have any interest whatever in Native American history, you should also visit the new National Museum of the American Indian. It's located across the street from the Air and Space Museum. Some people (including me) find some of the exhibits in this museum confusing, but it's a state-of-the-art facility dealing with a phase of American history that hasn't gotten enough attention. If you happen to be there around lunchtime, pay a visit to the cafeteria and try a buffalo burger.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.