I know rim to rim is the aspirational thing, but if you are going to GC, the Phantom Ranch experience is sublime.  I hiked the GC a few years ago with a friend who had reserved spots at the Phantom Ranch dormatory. As you go down it gets hotter and hotter and hotter and hotter until at last you arrive at Phantom Ranch where they have swamp coolers! and iced lemonade! and a steak diner!  So there you are just blissing out until about 3am when you have to get up to eat breakfast and walk up the hill on the other side.  We went North rim to South Rim, and when you get to the top of the Bright Angel Trail there is ice cream!  I have to say, the two-day version of the rim-to-rim hike is a peak experience because the pleasure points are so hightened.  You might want to enter the Phantom Ranch Lottery while you are at it.