I just made reservations for our family vacation this year. We are going to spend some time in and around Mackinaw City. We are going to spend a couple days at the state campground just over the Mackinaw Bridge in the upper Penninsula (the campground is actually ON the strait with a view of the bridge, which really excites me). Then we are going to spend a couple more days back on the lower side of the bridge at a KOA campground. On the "second leg" we will have time to actually go to Mackinaw Island. I've bookmarked a few things/places I want to take the kids to check out while we're there but for the most part we are just going to play it free and loose.
It shouldn't be a majorly expensive vacation either. Since we are staying in our camper at the campground our lodging will be about $40 a night. Plus we'll have the benefit of all the food we pack in the camper. I envision our major expenses being gas & souveniers.

I just love sharing these experiences with the kids (and judging by last year's trip they enjoy it too).