The White Rim Trail is Utah's ultimate multi-day party trail. The trail is a 103-mile loop on jeep road through Canyonlands National Park. Technically, this trail is fairly easy. But the length (and a few stiff climbs) means you need to be in good condition to do the whole thing. The major climbs add up to 4000 vertical feet, but an altimeter or GPS unit that records every little up-and-down will give you about 6000 feet total.
Most riders spend 3 or 4 days to ride this trail, spending the night at campgrounds. (Two days = Monster. One day = Lunatic.) You'll need a support vehicle for food, water, camping gear, and bike parts. (Even though there's little dangerous stuff, the length of this trail -- combined with sand -- means there's a good chance you'll need some on-trail repair.)
You can ride the trail either direction. Most bikers ride clockwise, from the Schafer Trail around to the Green River. This direction tends to be a little nicer, because it puts most of the sand on the downhill slopes. There are four big hills with over 1000 feet elevation gain, but you get to do one of them downhill as you drop into the canyon on the first day. A three-day trip gives you one major climb each day.