It is a LOT of driving. If you want to shorten it a bit, I would leave off the Grand Junction and Durango sections--even though the scenery is spectacular, they are pretty far away for a 2 week trip.
Hi! September is absolutely my favorite month up here--the weather is usually gorgeous, and our fall colors typically peak at the end of the month. One caution about only having 15 days--Colorado is
Aloha Jer, Kaena Point is a good hike, so is Diamond Head. As for beaches, the ones I prefer are North Shore and also Waikiki in front of the Moana. Then of course there are all the beautiful ones
Aloha Jer, Welcome to Hawaii. How nice to have your family come and visit :-) I am not sure if your parents are in good health and want to go on hikes, swim or rather shop and do "easy" stuff.
You are asking a hard question. I just spent two weeks in Jackson, The Tetons and Yellowstone. One thing that may influence your time is the weather. Be aware that it will be chilly at night. There