If you want to see the best and the worst of America, IMO you are starting with one of the worst. I would skip Niagara Falls. Instead spend more time in the western national parks: Yellowstone, Glacier, the big 5 in southern Utah, and Olympic NP are among my favorites. You might also want to consider the Canadian rockies. Definitely include the Oregon and northern California coast. Yosemite is OK but very crowded. Death Valley NP and the Mojave area are extra special but you need to make sure you are there in winter.