If you are going to spend a couple of days in Zion National Park, I'd recommend this one, right behind a trip into the Narrows, to get a great feel for what Zion Canyon is all about.

The trail starts right across the Virgin River (pictured) from The Grotto shuttle stop.  The stop has water and restrooms.

The first part of the trail runs near the Virgin River and is flat and exposed.  In the afternoon it is HOT.  Not only is there little shade but the canyon floor can really heat up.  Make sure to take plenty of water with you.