I read that the average stay is 4 hours which seemed strange to me

Yes, that does sound like what a friend of mine calls a "California
vacation" -- walk around a little, take some pictures, and get back
in the car. In this case, though, echoing some things others have
said, it is *so* big that you both can and can't experience it in a
brief visit or with one mode of transportation.

I fly a couple of times a month between the Bay Area and Albuquerque
on Southwest. Their nonstops go almost by the same route you would
drive, except for cutting a corner across Vegas. Weather and daylight
permitting, the view of the Grand Canyon* (and several other
picturesque bits of geology) is spectacular.

That's one extreme in how to appreciate this feature (and perhaps the
only way to get the whole picture), 15 minutes from up in the jetways
with a cup of coffee in your hand. You can also choose up-close-and-
personal ways at various levels of time and exertion that involve
hiking, rafting, or riding a mule, as well as lower-altitude scenic
flights. They all add up to a whole that only a lucky few, myself not
yet among them, ever have the privilege of putting together.

Anyway, as others have mentioned, there are many things to see and do
in the general area: Sedona, Prescott, Petrified Forest, Meteor
Crater, the Sunset Crater volcanic area, the Painted Desert (try to
catch it at sundown), the monument to the Code Talkers up at Kayenta
(in Burger King -- the cultural center of a small town isn't
necessarily where you'd first think to look), Glen Canyon Dam and
environs, Canyon de Chelley, and, oh, whatever serendipity blunders
you into. The only thing you can do wrong is rush.

Enjoy your trip (and perhaps think of it as a reconnaissance mission
for future ones),

--Joe

* Sometimes -- usually eastbound -- they opt for a more northerly
route over the Utah canyonlands and Monument Valley, no slouch for
scenery either.