Zion offers many options for hiking that range from easy to very strenuous. The most popular hikes are The Narrows, Angels Landing, and The Subway (requires extra permit).
This is definitely one of the most popular hikes in the park. It is a spectacular 25km gorge carved by the Virgin River. The gorge can get as narrow as 6 m wide and up to 610 m tall. Hiking The Narrows means hiking in the river. Most of the hike is spent wading through water and there is no maintained trail because the road is the river. Flash flooding can occur so check the conditions which change from day to day and are hard to predict before you start hiking.
The easiest way to experience The Narrows is to hike from the bottom and back. The shuttle will take you to the Temple of Sinawava and from there you need to walk around 1.5km to the mouth of the Zion Narrows. From there you will wade up the river. There is no formal destination so you can return at any poin, and return the same way you came.
The water level can vary in The Narrows. Under ideal conditions most crossings are around knee-deep but be prepared to have to wade through waist deep water, and in some instances even swim. Hiking boots with good ankle support are reccommended as well as walking sticks.