Located in Arizona, the Grand Canyon is a mile (1.6 km) deep and up to 18 miles (29 km) wide. Layered with bands of colorful rock, it reveals millions of years of geologic history on a scale no picture can fully capture. Watching the sunrise or sunset over the canyon is magical and not to be missed.
The National Park Service offers an excellent overview of the park, and Travel + Leisure has a helpful summary of when to visit and some of the attractions outside of the park. Amtrak has many vacation packages from all over the country if you are looking for a different vacation experience.