Hi I just got back from the Palm Springs area. Joshua Tree will be very hot in August. Plan to be hiking by 7:00 or so and be done shortly there after maybe by noon at the latest. Ryan Mountain is one of the best short hikes to do with great views of the park. Also Warren Peak is good. Both these are about 1200 feet of elevation gain at the most. The tram at San Jacinto is a great idea. It drops you at at just over 8000 feet elevation. from there, there are many hikes to choose from including the top at 10,800 feet. Also San Gorgonio which tops out at 11,500 would be cooler, but no trams to take you to that high starting point!
Palm Springs is a hiker's paradise, especially in the winter!

Have fun!
Garrett