What are the most worthwhile things to do in Grand Canyon on a first visit?

The South Rim's two maintained descents start within a few miles of each other and behave very differently. The Bright Angel Trailhead sits at 6,860 feet (2,093 m); Havasupai Gardens is 4.5 miles down at 3,800 feet (1,160 m), and the Colorado River is 7.8 miles along the trail with a rim-to-river elevation change of 4,460 feet (1,360 m). The South Kaibab Trailhead is higher, at 7,260 feet (2,213 m), and steeper: Cedar Ridge is 1.5 miles in at 6,120 feet (1,865 m), Skeleton Point is 3 miles in at 5,220 feet (1,591 m), and the river is 6.3 miles along with a 4,860-foot (1,480 m) change. The shorter trail is the harder one, which is the single most useful thing a first-time visitor can know.

From the North Rim the arithmetic changes again. That trailhead is at 8,241 feet (2,512 m) and the North Kaibab Trail runs 14.2 miles to the river with a 5,850-foot (1,780 m) elevation change — nearly twice the distance of its southern counterparts.

At the eastern end of the park road stands Desert View Watchtower, a circular stone tower 70 feet tall, designed by Mary Colter and completed in 1932. It is a National Historic Landmark in its own right, and the East Entrance beside it is open around the clock, which makes it a workable arrival or departure point rather than just a stop.

The World Heritage criteria behind the park's listing — (vii), (viii), (ix) and (x) — point at four separate reasons to look: scenic value, the geological record in the walls, ongoing ecological processes, and biodiversity. The layered rock you see from any overlook is the reason criterion (viii) applies, and it is the thing a short descent actually puts you inside.

Walking down from the rim, the vegetation changes within the first mile as the trail drops out of the pinyon and juniper of the plateau. On South Kaibab the route follows an open ridge rather than a side canyon, so the view is continuous and there is almost no shade — the reason that trail reads harder than its mileage suggests.

The Park Service renamed Havasupai Gardens in recognition of the Havasupai people who farmed there, and the name on the official trail-distance table is the new one. Older maps, older guidebooks and older trail signs still carry the former name, so a first-time visitor comparing sources can end up thinking there are two separate places at the 4.5-mile mark. There is one.

• Down and up — Turn around at a published landmark rather than a clock reading; Cedar Ridge at 1.5 miles and Skeleton Point at 3 miles exist precisely as turnaround points.
• East in the afternoon — Drive the eastern rim toward Desert View later in the day, when the Watchtower's elevation and the low sun work together.
• Arrive or leave by the east gate — With the East Entrance open 24 hours, the Desert View side is the flexible end of the park.

The elevation change is the real constraint. A descent of 2,040 feet to Skeleton Point has to be reversed, and the return is the hot half of the day on a trail with no water source. The park's corridor trails have been affected by flooding and water restrictions in recent seasons, which can close sections and inner-canyon facilities without notice. The historic rail approach to the South Rim runs from Williams across Kaibab National Forest land rather than through the park, so conditions outside the park boundary can shape how you arrive. Anyone with knee or altitude concerns should treat the rim trail, which is close to level, as the primary walk rather than a consolation.

The Grand Canyon when-to-visit question in this group sets out the North Rim's operating season, which decides whether the 14.2-mile northern descent is even an option for your dates.