The South Kaibab Trail descends into the Grand Canyon from near the Yaki Point overlook. The full trail drops 4,700 feet over a very steep and strenuous 7.3 mile route to the river that requires at least 2 days for a round trip. The park advises very strongly against attempting the trip in one day, but despite this, every year about 100+ young 20-something mostly male hikers must be rescued due to heat exhaustion, dehydration and other complicating factors.
Day hikers, however, can access the upper portion of the trail for some spectacular views. Short (though still strenuous!) hikes can be made to the Ooh-Aah point (3 miles round trip, 900 feet down, 900 feet up) and the nearby Cedar Ridge viewpoint (approximately 4 miles round trip, 1500 feet down, 1500 feet up).
This picture was taken half-way between Ooh-Aah and Cedar Ridge.