What are 'don't miss' sights to see when travelling around the USA?

Right! I’m travelling around the USA this summer. I’m starting in New York, then up through New England, to Niagra, Washington DC, Chicago, Wisconsin then across on the I90 through Yellowstone to Seattle, down Highway 101 through San Francisco, LA and then to Vegas. THEN past Grand Canyon then to San Antonio, then New Orleans up to Memphis, back down to Atlanta and then down through Florida hitting Jacksonville, Orlando and Miami.
What should I make sure I check out along the way?!