The Narrows Zion
This is the most popular hike in Zion Park, and one of the world's best canyon hikes. It is pure fun and can be tailored to suite any ability level. The trail is the Virgin River. The canyon is so narrow, the river covers the bottom in many spots and you've got to wade or swim to proceed. Plan on being wet. In fact, the cool water makes this hike particularly pleasant during the hot months of summer.
Temple of Sinawava(37.285, -112.947)
Most casual hikers start at the bottom, hike up the canyon as far as they feel comfortable, then return the way they came in. The hike begins at the Temple of Sinawava, Gateway to the Narrows, at the end of the shuttle route in Zion Canyon.
Into The River(37.296803, -112.948238)
From Temple of Sinawava, the paved Riverside Walk leads up the canyon for about 1 mile. After that, the river becomes the trail - you just hike/wade up the river.
Orderville Gulch(37.3078, -112.946)
Orderville Gulch is a prominent side canyon about 2.5 miles into the canyon. Most casual hikers don't make it this far, which is sad because the canyon here is very narrow and very beautiful. You can explore up Orderville a ways, if you want. An option for Canyoneers is to start the hike at the top of Orderville Gulch, hike down it and then down the lower narrows, and end the hike at Temple of Sinawava. (Orderville is a semi-technical slot canyon that requires short rappels, serious scrambling and much wading. A permit is required to hike it.)
Chamberlain Ranch(37.385, -112.839)
For the complete adventure, hike the entire Narrows from top to bottom, from Chamberlain Ranch down to Temple of Sinawava. You'll need to shuttle to the trailhead, located along the North Fork Road, on private property on the east edge of Zion Park. A permit is required to high the entire Narrows.
The top to bottom route is 16 miles. Wading slows progress and so this is best done as an overnighter. Strong hikers can do it as a long day hike.