It was closed when we arrived, but was beautiful from the outside. There was a wedding party taking pictures on the beach behind it - bet they turned out great :)
The view was lovely, though not the darkest sky-viewing area I've seen. I'm sure part of that was due to being at the edge of a lake, a big reflective surface. The staff on site was wonderful though, and were available to answer all kinds of night sky questions. They had set up a high powered telescope near to the beach, and allowed the public to take a look. We got to see the rings of Saturn, thanks to their help. *Very* cool.
Note: the drive in was fabulously creepy, as the only car on a narrow winding road through a pitch black forest. We will go again next time we visit.
We visited Castle Rock and made the climb - totally worth the effort and the $1 entry. The view from up top was wonderful, with forest and lake Huron sprawling out in front of you, and the Mackinac Bridge visible in the distance. As a bonus, you get to visit Paul Bunyan and Babe the Blue Ox, they sit at the base of the stairs.
When you come back, they will give you an "I climbed Castle Rock" sticker :) We thought it was pretty cute, and we felt we earned the sticker after all those stairs.
The museum has been updated heavily since the photo given on this page. It's well put together and presents cultural artifacts in an engaging way. There are two video presentations available for viewing as well. There is no charge to walk through the exhibits, but they accept donations if you're so inclined. There is a small gift as well, it's worth a look through.
This bridge is 5 miles long! It was absolutely amazing. Possibly of use for those thinking of visiting: there is a $4.00 toll per car (higher for larger vehicles, obv). Very much worth seeing.
This is the Days Inn Bridgeview location - there are two in Mackinaw City. It's cute, built to look like a log cabin. It's clean, comfortable, and has the minimums you'd expect for the price with only one exception... do NOT count on the Wifi. It was there, but the signal was so weak as to be unusable the entire time.