I've visited the US a couple of times now. I've stayed in Seattle, San Francisco, Boston, and Washington.
After spending three nights in San Francisco, I caught the train across the country to Boston - I hate flying, so I don't mind spending three days to get across the country, and you do get to see the country out of the window!
Two highlights of my journey across the States was my bicycle ride over the Golden Gate bridge, and my trip into the bay to visit Alcatraz.
I travelled on the Acela train down from Boston, through New York Penn station and into Union Station Washington.
If you find yourself in Boston there is one thing you MUST see, the "Mapparium" it's a huge glass globe that you can walk inside, and see a map of the world on the inside of the globe. It was created back in the 1920's, so there are a few of today's countries that don't appear on the surface - and there's a lot of Pink for the British Empire.
I also got to see a baseball game in Baltimore - The Orioles at home against the Indians.