I could see an overnighter because you're right, there's a lot to squeeze in for just a day trip. Plus you'd be up early to catch the 7:20 ferry from Charlevoix and then come back at 5:20pm, not a lot of time and you would need a car for much of it. Split 4 (or more) ways getting a car over wouldn't be too bad and you'd be back in time to help kill a bottle of Maker's Mark. ;-) The problem with an overnighter is on might potentially be paying for two hotel rooms for 1 night. I'm split, I can see benefits and drawbacks for both but I'd lean towards an overnighter just because it's $150 to get a car there and back on top of $24/person. If you offer it as an option, I'd be up for showing people where some neat stuff is and being a bit of a guide.