If you've never been to Moab before, you've never been to Dead Horse Point. That is number one. Whatever it takes, you have to go to Dead Horse Point State Park. Ideally, you'd arrive there a half hour before sunset (or a half hour before sunrise!). If sunset, you'll drive back to Moab (an hour, more or less) in the dark. If it's impossible to arrange a visit to this location, you'll just have to go back some other day. The drive to Dead Horse is wonderful, the point itself is a religious experience. IMHO, better than the South Rim of Grand Canyon.
Number Two, as you suggested, Arches National Park. It's virtually downtown. If you can't get to Dead Horse, go to Arches twice. Go as far into the park as the campground, stop lots and plan on at least three hours per trip. If you can arrange it in early morning, lucky you. At any time of day, it's an incredible landscape.