Only because it's August should you skip Moab. It will be HOT. You will be missing the opportunity for a couple of great roads though. More on that in a bit.
If you come down to Grand Junction and your goal is to head east from there, go east on I-70 to just past Palisade and look for CO 65 south or off to the right towards Mesa, CO. This is an amazing twisty road that starts right off I-70 into a series of canyon walls and great bends, then eventually ascends to the Grand Mesa at 10,000 feet in a heavily forested paradise. It will be cool on top, even in August, the opposite of Moab. You will come down into Cedaridge, CO and continue on to the junction of 65 and CO 92, just east of Delta, CO. It's a large T intersection. Turn left on CO 92 headed for Hotchkiss, CO. In Hotchkiss, turn left onto CO 133 north towards Carbondale, CO. This is also a truly awesome and often, empty road. You'll see the beautiful interior of untouched Colorado. Except for the coal mine
From Carbondale, you can simply go north up to I-70 at Glenwood Springs and catch Glenwood Canyon on I-70 en route to Denver, or if you have time, you can do more zig-zagging your way back. Keep in mind that I-70 east into Denver can, itself, be way more congested in August than back roads. It can turn into a parking lot; I'm not kidding.
More possibilities: Go back to I-70 east, but in Minturn, go south on US 24 towards Leadville. The climb up 24 past the old Camp Hale 10th Mtn. Div. is old, remote, and wonderful. If you want to loop down through Colorado Springs, stay on 24 all the way to I-25 and back to Denver. Or, when you get to the junction of 24 and CO 91, turn left and go back to I-70 at Copper Mtn.
Another similar and longer route taking you around to either Leadville or going south to Colorado Springs is to leave Carbondale heading south to Aspen and up over Independence Pass. At the bottom, you can return to I-70, or go south on 24 to CS. Independence Pass is pretty cool.
In the end, if you choose I-70 all the way back to Denver, don't miss CO 103 out of Idaho Springs taking you up to Echo Lake and Mt. Evans. This loop rivals the curves and scenery of the others, but it will be a little more crowded, especially if it's the weekend. Lots of bicyclists. It's not as long as the other loops either, and you'll be back to I-70 in reasonable time. All these roads are of course, less crowded on a weekday. If you have the time to meander a bit, I'd take one or two of the loops. If you can only do one, take either 24 up to 91, or 103/Mt. Evans. Those are the shortest two.
If you do go to Moab, enter off of I-70 onto UT 128 south. Amazing. Pass through Moab to La Sal Junction via 191 then turn left onto UT 46 back into Colorado (becomes CO 90). At CO 141 (middle of nowhere), turn left on to CO 141 back to Gateway and then Grand Junction. Another amazing road, but it could be sweltering through here; it's desert canyons.