Mackinac wouldn't be my first choice for a 9yo. It is fun, but more of a subdued type of fun. If you do want to go there, I'd suggest looking at Mackinaw City hotels. Some of them have indoor and/or outdoor waterparks or at least slides now, and in the waterfront tourist area there are some fun things to do in the city.
Zehnders is really fun now that they've got the indoor waterpark and they're a lot cheaper than Kalahari & Great Wolf. On our first visit there we had the most perfect room, a family suite with a balcony that looked out over Bronners. There is something so magical about sitting on the balcony on a warm July night, watching the Christmas lights twinkle.
And Frankenmuth is just such a fun town to explore, with all the little shops, the awesome fudge, the old-fashioned arcade, etc.
You might also want to look at Boyne Mountain, depending on where in Michigan you're going to be. There's not as much to do around there unless you're/they're outdoorsy types, but they've got a nice sized indoor waterpark and they're smack in the middle of some of the most beautiful scenery I've ever seen.
On the west side of the state, there's the Double JJ ranch which has an indoor waterpark, outdoor waterslides/pool, and horseback riding too. It isn't too terribly far from Michigan's Adventure amusement/waterpark either, so you could spend a day there if the girls want or just hang out at the resort.