Yellowstone N.P is a huge caldera & volcanically active area. The Rockies are stunning to cross & Yosemite N.P in the Sierras in California is stunning. San Francisco bay is beautiful as are the nearby redwood forests. Lake Powell and its canyons are beautiful as are the trio of canyons Bryce, Zion and the Grand Canyon. Loads more to see there like Arches Canyon, Monument Valley & Death Valley that I haven't done. Crater Lake in Oregan, another caldera, is beautiful. Washington State has tonnes to offer such as Mount St Helens, Mt Rainier and the cascades. Offshore Hawaii has the volcanos and tonnes of beautiful sights such as 'the Needle' on Maui & Alaska would be well worth seeing. Over east, which I haven't visited, is Niagara Falls and the Everglades in Florida. Perhaps others who are more familiar with eastern America could fill you in better. about 6 years ago
A very broad question - the United States is vast.
I've always been a big fan of the mid-west region, where you'll find a lot of the places Hilary mentioned - Bryce Canyon, Zion, Grand Canyon, Monument Valley etc.
A road trip out of Phoenix, AZ, heading north will take you into serious natural wonder territory. about 6 years ago