As others have suggested, a return via Route 12, Cap Reef, etc., is spectacular, especially in the Fall, but it is a long drive. An alternative route if you're interested in shooting Native American rock art, is the Rochester Panel near Emery, Utah.
As for St. George, there are things to shoot here. The small and compact Snow Canyon State Park is about as fine a display of red rock as you'll find anywhere in such a small space. I find it far more interesting than Kodachrome Basin, as just one example for comparison. Lodging on St. George Blvd. in St. George will be less expensive than Cedar City. If you time your travel correctly, you could leave Moab in the morning, easily be at Kolob Canyon for sunset and then motor on down to St. George for the night, shoot sunrise in Snow Canyon and head on to Las Vegas the back way via old Hwy 91, putting you back on I-15 just east of the Arizona/Nevada border.
If you really want to explore some possibilities, I would suggest you pick up a copy of Laurent Martres' PHOTOGRAPHING THE SOUTHWEST: UTAH edition. It is pretty much the reference standard for identifying locales, time of year/day, lenses to bring, etc.