Gary, we love Yellowstone. It is so unique and beautiful. Both times we were there was in September.
For the first trip in the fall of 2002, we took my husband's baby--his 1996 Corvette. What a trip! We had to pack very sparingly in order to fit everything in the Vette. But we made it and have some gorgeous pictures to show for it.
You will enjoy the trip. Anytime spent with kids and grandkids is very special.