If you are going this year, be aware of construction in Arches NP. They had the park closed from 7 pm to 7 am Sun through Thursday when we were there a couple of weeks ago.
If you want scenery more than a wild 4x4 ride, try the White Rim Trail in Canyonlands. Doable in a day and really a fantastic experience. (Some of the hard corp mountain bikers actually ride the entire 100 mile loop in a day :E )
People have already mentioned our favs around Flagstaff: Wupatki, Walnut Canyon, Sunset Crater. Sedona can be downright hot in mid summer. Plus pretty busy. Oak Creek Canyon north of town is a surprising respite from the heat.