The Maroon Bells should be lovely in early August. Crowded (by Montana standards), but lovely. There's more good summer hiking in SW Colorado than you can shake a stick at. The Weminuche, etc.
The Needles area and Custer State park are much more interesting than Rushmore. You'll kick yourself if you give them a miss.
Bryce will be pretty hot in August. Cedar Mesa will be REALLY hot. The Below-the-Rim Trail is pretty nifty, especially the southernmost sections, and you ought to have it mostly to yourself. Water is scarce and you'd have to carry a fair bit, but won't need much insulation so it works out in the end.
The heat of summer makes hiking in wet canyons very nice. An overnight trip in the Narrows of Zion, for instance, lets you play in the water and enjoy the wading without getting cold.
The higher mountain areas of the Colorado Plateau are also very nice in high summer. The Aquarius Plateau north of Escalante is gorgeous, and very little traveled. Few trails, get a map, drive up, go out and explore. Good fishing in the lakes.
The north rim of the Grand Canyon is not far away either, and the high forest there is lovely in summer (or anytime of year).
Should be a good trip.