Definitely Platte River - we volunteered there for nine seasons! :)
Sleeping Bear Dunes has awesome beaches and the gentle Platte River gives a safe canoe or kayak ride. The rental can be walked to from the campground - along with an ice cream cone.
The whole area has unique quaint small towns to explore. It's a gorgeous area. You can also take a ferry to South Manitou Island for a day trip and tour the lighthouse (we lived on the island three season to give the tours) and there's also a farm tour of the early inhabitants. No stores so bring your drinks and lunch.
We've never had a mosquito problem and flies were bad only for a few day period one year.
It's an extremely popular park so get your reservations now.
