James:

Lots of options, but it will definitely be hot !!!

North Rim of Grand Canyon - 275 miles
Zion National Park 160 Miles
Death Valley NP - 150 miles (But you been there & done that often)
http://www.grandcanyonskywalk.com/ -120 miles (This gets my vote)
Valley of Fire State Park -55 miles (Not for the faint hearted in August)

Jim