Boston

Boston needs 3 days: the Freedom Trail (16 historic sites from the American Revolution) on one, Harvard and MIT in Cambridge on another, and the Back Bay neighborhood on the third. May-October is the widest window, with fall (September-October) standing out for foliage colors. It's expensive, among the priciest cities in the northeastern US, partly due to its large student population. It's the birthplace of American independence (the Boston Tea Party, 1773) and home to some of the world's most prestigious universities, giving it a unique academic profile among major US cities.