The Yellowstone is the oldest National Park in the USA and hosts a wide variety of animal life (incl. bears and buffalo) and vegetation. The natural beauty will make quite an impression. Beneath the Yellowstone there is volcanic activity that causes many hot pools and geysers ("Old Faithful").
The Grand Canyon is really grand. When standing on the ridge of the canyon and looking 1 mile (1600m) down to the river Colorado that created all this, you do owe the river a lot of respect. It is possible to walk all the way down and back up in a day, what can be recommended.
Grand Teton National Park is a great park not far from Yellowstone.
Mount Rushmore National Memorial is centered on a colossal sculpture carved into the granite face of Mount Rushmore in the Black Hills, South Dakota. The sculpture features the heads of Presidents George Washington, Thomas Jefferson, Theodore Roosevelt and Abraham Lincoln.
Zion National Park is one of the great national parks in Western USA featuring geology, basins, rivers and walking ways.
Las Vegas is the show and gambling capital of the USA. World famous and featured in many, many movies. A wide variety of hotels from budget places to top-class resorts.
Monument Valley is a desert with few small, steep mountains/rock formations in the shape of monuments. The area is a "classic" and must-do in the Western USA. Of course it is featured in many movies...
Niagara Falls are one of the top 3 waterfalls worldwide in regards of size and name. The falls are located at the border between the USA and Canada, west of Buffalo. The Canadian Horse Shoe Falls are the most famous and visited by many tourists and honeymooners.