Zion National Park is among the most popular (2.5 million visitors a year) of the United States' national parks for several good reasons. In practical terms, it's one of the most accessible, being an easy two-hour drive from Las Vegas, NV. The largest city in southern Utah, St. George, is practically at Zion's doorstep just 40 miles west, providing an excellent selection of stores in which to stock up on the basic goods before you hit the trail. Boasting 300+ days a year of beautiful sunshine and blue skies, this park can enjoyed during all seasons. And with visitation in the peak months of July and August topping 11,000 per day, you might want to consider coming during the other equally beautiful and considerably cooler months.
Zion is exceptionally friendly to bicycle travel, with the paved highway through the Park as well as one trail (Pa' Rus, 3.5 miles along the Virgin River right through the valley; the trail is flat and gorgeous) being open to those of you willing to pedal. In fact, in the summertime, the Scenic Drive is open only to bicycles, chartered buses, and the park's exceptional clean fuel shuttle (which is free). After years of build-up toward smog, traffic, and snarling visitors and rangers alike, Zion opened its shuttle system in 2000, which has dramatically reduced all sorts of vehicle-related problems. The shuttle also stops in several places in the immediately adjacent town of Springdale, making both staying and eating in town a viable option for travelers.
Springdale is an added bonus to the Park's popularity with many different visitors, since those who don't wish to have a strictly wilderness-based experience can retreat to a soft bed, hot shower, and really good meals each night if they want, not to mention a giant screen movie at Zion Canyon Theatre. There are also several guiding companies located in town that offer great package deals so you can really get the most out of your visit. If you can, plan to spend a bare minimum of two full days in Zion in order to get a taste of some of its best offerings. Zion is often part of a "loop" tour that many people do, which can include venturing on to Bryce Canyon, Capitol Reef, Arches, and Canyonlands National Parks, in order to get an intensive American Southwest experience. Zion is a great place to start off, and it has so much to do and see—take advantage of every minute you spend in this gorgeous, unforgettable place.