My crystal ball says... JK
I would expect nice days with cold night. The water is always fucking cold, so on big water days wear a dry suit. In the summer you always think "where is the shade going to be" and if i were you I would think "where is the sun going to be", so then camp will be a little more comfortable.
There is a nice camp a couple of miles below Red Wall in the corner (sorry i forgeet the name) and then just below that is South Canyon (great hike). another good camp is at mile 75 (rattle snake) good sun. You could stop at the Unkar Delta and check out the reuins, cross the river and hike up to see the view of the river and to look directly down on Unkar (SWEET) blah blah blah i could go on.
Have you run the Grand before? How many people? Where do you want your lay over days to be?