The Glen Canyon National Recreation Area has seveal good mountain biking routes. There are great slickrock singletracks with views of Lake Powell and the Grand Canyon, plus a nice swim to finish your mountain bike ride.
To reach the Glen Canyon National Recreation Area from Flagstaff, head north on US89 to Page. Once in Page, it is highly recommended to stop at Red Rock Cyclery for a free biking map along with directions to the mountain bike rides.
The mountain bike routes range from easy to technical depending on what exact route you choose. The loop is 10 miles long, with a three mile side trip on the the slickrock to Lake Powell. It is a great training mountain bike ride, the single track has been described as a giant BMX course on the edge of a cliff with killer views. The slickrock is more difficult than Moab in places, but mostly very fun and ridable by intermediates. This loop is the course of the Red Rock Rumble MTB Stage Race cross country.
Overall, Glen Canyon National Recreation Area is the 31st most popular mountain bike trail of all 103 mountain biking rides in Arizona.