You should visit the park whenever you can, any time of the year. In the winter the North Rim is accessible via skis, but the South remains open and relatively uncrowded (with occasional snow). Some campgrounds and parts of lodges may be closed through the winter and there are fewer people making day trips from Flagstaff and Williams. Of the units in the park service called National Parks, the Grand Canyon ranks #2 in visitation with about 4 million visitors per year -- behind the Great Smoky Mountains (with no hotels in the park and no admission fee, there is a lot of day traffic from the gateway cities) and ahead of Yosemite and Olympic. Even in the peak of the summer, the drive east of the village area is relatively uncrowded with parking at the viewpoints. Visitor parking is greatly improved with the expanded parking near the visitor center and the closing of the Mather Point lots. Parking in the village area is always difficult and the shuttles can be full, but it is worth the wait.
There are 3 main shuttle routes: The Hermits Rest (Red) shuttle starts in the village near the top of the Bright Angel Trail. There are fewer stops on the return route than the trip to Hermits Rest. The Blue Village Shuttle loops through the village area from the Red shuttle transfer to the Visitor Center (hotels, market, campground, etc.). Due to the loop, it is better to walk between some locations. Choose your stop and direction carefully. The Orange Kaibab Shuttle links Yaki and Mather Point (which have no parking or car access) to the Visitor center. These operate 12 months a year (except the Red shuttle is only March-November). The shuttles help in planning walks to maximize your viewing pleasure.
Generally people arrive at the park by driving, but there are other options. From Williams you can take the one day train excursion and avoid the parking problems in the park. You are at the rim for part of a day so the shuttles cover everything you would be able to see in that time (though people who afford the train often take one of the tours, which avoid the shuttle lines). Shuttle service to the park is also available from Flagstaff (with Amtrak service and an airport) Arizona Shuttle or Grand Canyon Shuttles and the very important (for trans-canyon hikers) Rim to Rim Shuttle. The free Shuttle to Tusayan (the concentration of hotels on the Flagstaff/Williams road near the park boundary) to the Visitor Center operates through the summer. Bike access is good, even if it is a long ride to the next campground. Also, you can ride your bike on the West Rim drive which is closed to cars most of the year.
The Official Grand Canyon National Park site has all the offical information, including the pointers to accomodations, camping, hiking, news, traffic, weather, shuttles, etc. Why look anywhere else when this is the current information. Check the online news for any changes and links to accomodations.
Some people like the luxury of the El Tovar, but the Thunderbird and Kachina Lodges have the same views and are just as close to the rim. Bright Angel Lodge includes historic cabins near the rim. Maswik is within walking distance to the central village facilities (given the direction of the bus loop, it is faster to walk to the rim, but take the bus from the rim) and is cheaper, but is your typical motel-like facility. It is a relatively efficient use of space. Yavapai Lodge is closer to the store, closer to the headquarters buildings (where the evening programs are held) and a short walk to the rim for good sunrise views. All these are managed by Xanterra Parks & Resorts
Mather Campground is near Yavapai Lodge (and the main store). Desert View Campground is near Desert View (obviously) and the eastern entrance. There are many motels just outside the southern entrance on the road to Williams and Flagstaff (at Tusayan, with regular shuttle service in the summer months to the visitor center). To the east there is Cameron, with a few (2?) motels. The Lodge on the North Rim (managed by Forever Resorts) is the only indoor accomodations there, with outside the park options in Jacob Lake and Kaibab Lodge (part way to Jacob Lake). Campground reservations are handled by the usual National Parks Campground Reservation site.
Most people just visit the Village area and some claim that studies indicate they spend more time in the gift shop than on the rim looking at the canyon. Arrive early, have reservations, spend 1 or 2 (or more) nights, take the shuttles to all the view points, walk along the rim, walk down the trail -- but be careful walking down, don't go too far since you have to come back up and water may not be available. For some it takes twice as long to come up as go down, for others it is about the same time. (It is generally more dangerous to fall while going down than going up, so be careful.) In the winter when it is icy near the top, the walk down can be much slower to avoid sliding over the edge. And in the evenings, consider attending a Ranger Talk.
In an effort to clean up waste, Grand Canyon does not sell water in disposable containers. You should bring you reusable water bottle and refill it at the markets, lodges, or visitor centers.
There are a number of books you may want to read. Colin Fletcher's book is always interesting The Man Who Walked Through Time: The Story of the First Trip Afoot Through the Grand Canyon for a view of the park few people see. His later book: River : One Man's Journey Down the Colorado, Source to Sea is about more than the Grand Canyon, but provides a different perspective on this region.
A River Running West: The Life of John Wesley Powell by Donald Worster is an account of the exploration of the Colorado River by John Wesley Powell.
A unrelated book, but one that is fun for kids and is about the Southwest is Danny Diamondback.
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Maintained by Keith Price. Also visit Yosemite National Park or Sequoia and Kings Canyon National Parks