The only trail to cross the world famous Grand Canyon, Arizona’s Kaibab Trail is challenging (one walks down first, then up -- “mountain climbing in reverse”), but richly rewarding.  It features fascinating geology as well as five (of seven) North American life zones with their distinctive flora and fauna.  Hikers cross the powerful Colorado River by bridge and will likely see boaters rafting the river.  This is primarily a backpacking trip, though some hikers stay at Phantom Ranch near the Colorado River.