James:

Looks like the Canyon is your best option. The elevation at the North Rim is over 8,000 feet. Even in August, daytime highs run around mid to high 70's. It even gets chilly enough to need a jacket at night or for early morning sunrise shoots. The South Rim is about 8-10 degrees warmer.

There are some mountains within an hour or two's drive from Vegas, like Red Rock Canyon & Desert NWR, but I am not familiar with them. I do know that August is not a good time for migratory birds in this part of the country.

Jim