We did a lot of camping this year, but we really loved the remoteness of Brighton State Park in September. It was too cold for swimming, but the leaves were just starting to turn. We stayed in the Poplar lean-to, which was very private, with a short trail leading down to the lake. Some of the other sites are right on the water, which would have been nice too!
Great trails right in the park, including the Shore Trail and the Boreal Forest loop. There are lots more trails to discover just a short drive from the park. We visited just as the foliage season was getting underway, end of September (right now) is peak this year.
100% recommend this campground and am already planning next year's trip!