You have to see the Grand Canyon. It's one of the very few sites in North America that justify use of the word "awesome."
But when you do get to the South Rim, the hub of Grand Canyon National Park, you also have to expect that you won't be alone. This video shows the canyon's beauty and heavy tourist traffic all at the same time. (Because the video is accelerated, the tourists look not only tiny but very busy. In a word, antsy.)
As the video notes, in its first year as a national park, in 1919, the Grand Canyon drew about 37,000 recreational visitors. The park gets that many in three typical days now.
For more on the canyon -- a lot more -- check out this package of photos, stories and video.