We were in Moab and Monument Valley in August 2015. As mentioned, if you are going hiking at Arches, get out in the morning. We generally went out about 8:00 and went for 3+/- hours. A bit toasty toward mid-day, but not a big deal.
Do the jet boat on the river -
Moab Jet Boats -- you'll get wet and the water is cool.
Also, our favorite was driving a 4x4 on a tour of Hell's Revenge. It was soooo much fun
T- Rex rental
Joe
2013 Dutch Star 4338- all electric
Toad is 2015 F-150 with bikes,kayaks and Harley aboard