Thirteen years ago, my husband and I stayed at the Grand Hotel to celebrate our 20th anniversary. The date was October 1, and the weather was absolutely perfect! We enjoyed sunshine, blue skies and high temperature of about 70. We biked all around the island, and were pleasantly surprised by the lack of crowds. We had paid for an interior room at the Grand (it's all we could afford), but because they were not nearly full, they up-graded us to a lakeview room at no extra charge!
That was 13 years ago, and last year we stayed again, this time in May. The weather was cold and windy on the first day, but sunny and warmer on the second day. A small shop owner said the weather seems to be shifting to spring coming later, and fall lasting longer. But, as any Ohioan knows, you can never count on the weather.
I will say that the fall colors we viewed on our trip from Ohio up I-75 to Mackinaw City, were the most spectacular colors we had ever seen, especially once we got north of Standish, MI. Again, every year is different as far as leaf color brilliance.
As for romantic suggestions to do, just GO. The romantic atmosphere of the island itself, and especially the Grand Hotel can't be beat if you love quaint (and few kids, if you go during the off season) We are going back in October, so maybe we'll see you there!