I have stayed at a Wisconsin lake house several times, which is pretty darned close to Michigan so I will weigh in. I'm going to guess you are visiting in August or later due to the timing of your post. So I would suggest a set of sweats/hoodies for each family member. Nice to cuddle up in for campfires on cooler August nights. Close toed shoes are good for long walks on the beach. I hate sand getting wedged up in my toes while wearing flip flops. Easy slip on shoes are important for the kids so they aren't tempted to run around barefoot. And several types of bug dope and better yet a thermocell are essential. The skitters up here are terrible!
As for toiletries, I pack a small plastic laundry type tote (similar to what the college kids use for the showers) with my shampoos and soaps. It goes right into the showe with me and the holes allow the water to drain out. I put my non-shower creams and makeups in a separate zippered toiletry bag.