Man I could write a few pages on this. But with August you are somewhat limited. First of all get a National Parks Pass - although I think they call it something funky this year and they raised the price from $50 to $80. If you go to more than one park it pays for itself.
Sounds like Rocky Mt. Natl park would be a good place to start. IF you wind up coming SW be sure to check out Zion and Yosemite. Although Zion will be fairly hot and Yosemite will be crowded, but so will everywhere else that's not scorching in August. Best time to do the Zion Narrows river hike is August - it's awesome. To me one of the highlights of So. Utah is just taking the backroads all over the place, and as long as you have AC, you can do this anytime. Bryce Canyon is a lot cooler than Zion as it's at higher elevation, and definitely worth seeing.
Here's my trip report and pics from that area which you may find helpful:
http://www.synergizethis.com/FourCornersTrip.html
Also there's a link to some CA deserts stuff off that page.
And Yosemite is such a magical place, I can't imagine ever having a bad time there. If you do Yosemite - try hiking from Toulame Lake (sp) up to Cloud's Rest. But be sure to train first, the first couple miles are fairly brutal. But once you get to Cloud's rest and the day hikers have gone back, you'll have the place all to yourself. In August there's a good chance you can sleep out under the stars, which is a really magical experience. Then from there you can hike down towards the valley and do half-dome - if you have any legs left. Hiking to the top of Half-dome after Cloud's Rest is probably the most worn out I've ever been in my life. But I made it, and I'm a fat POS. So a lot of people should be able to.
Feel free to PM me if you have more questions.