There's always the option of checking out the nearby La Sal Mountains which have peaks topping over 12,000 feet. Looking at the map, Canyonlands' Island in the Sky tops out at just 6,000 feet so if you're really set on hitting the Moab area with slickrock and sandstone experience , you could camp at any the Island in the Sky or Dead Horse State Park campgrounds. No solitude, but the views are killer from up there and 70 degree night time temps up there wouldn't make for terrible sleeping. Arches is at 4,500 at it's lowest and 5,000 at it's highest. Everything in Arches is short and relatively flat so I think early morning hikes in the park would be just fine in August. Really, I think it depends on how dead set on doing longer and challenging outings. If you really want to get into the backcountry, wait until October. If you're content doing short and light hikes, which are all that is required to see all the really famous sights up there (Delicate Arch, Landscape Arch, Turret Arch and the Windows, Double Arch, Grandview Point, Mesa Arch, Dead Horse Point, ect), then an August would be a little toasty but still worthwhile. I went up to Moab for a three-day weekend this past November, and did just that. I had never been up that way so I was stoked just to see all the landmarks, and of course I'm itching like mad to go back and explore further. Oh, and if you do go, check out Corona Arch.
I'm standing alone on the cliffs of the world
No one ever tends to me
Sitting alone, covered in rays
Some things are so my mind can breath