If you or anyone in your family is even the slightest bit of a baseball fan, take a tour of Fenway park. They're offered many times, even when the Sox are in town.
Coming in late September? Rent a car, and drive an hour north of Boston, into New Hampshire, and bring your camera. The foliage should just be starting to turn up there around then, and words simply don't do justice. I've seen foliage in most parts of the country that have something to offer, and this is as good as it gets! The more off the beaten path you go, the better it gets.....
September is absolutely one of the best times to come up here...enjoy!