If you are going to the GC be sure to check the weather. We were dressed for Phoenix and it actually snowed a little while we were in Williams. By mid-morning it was normal temps, but I was not prepared for the light dusting in the morning. Sedona was fine the entire time BTW.
Dude, not my first rodeo! Elevation determines the temperature.
Edit - Mike we are much more aware of what to do in the west, and what gear is needed. Spent the 90's and the early 2000's knocking around Northern AZ and Southern UT. Every chance we could get, we were doing something out there. Just saying we know better now. Some guys at work used to call me the Dessert Rat, as we were always there.
In 1984 we left Pheonix and went to the south rim, in shorts and short sleeve shirts, and we're fine in the daylight. Left the Canyon at dusk, stopped at a Fast food place in Flagstaff on the way back and we were darned cold, and had the "must be tourist look" from locals.
We have been to the north rim in summer, and since it is 1200 ft higher than the South Rim, it can get a little nippy at night.
If you have never seen the Canyon from the bottom on a river trip, it should be a big bucket list item. I want a repeat of that one, 15 days on the river, it was the best vacation ever!