Nice trip. You will be an official “leaf peeper “ when you are done. Depending on how long you are on Cape Cod you are close to the ferry to either Martha’s Vineyard or Nantucket. White Mountains and Stowe should be in peak September. Packing will be a challenge as you may hit high 80’s to low 30’s.

I know you are booked on everything but if I was to change one thing it would be to skip Boston and go up 128 from the cape and stay in Rockport. I don’t like city stops on road trips. Too much hassle with the car. Just my thought.