August. I’d look at New Buffalo or St Joe. If you want to do shopping and dinners. Not too far to drive. August is very busy as far as rentals go. I’d look for a place to stay and go from there.
FYI for others. Glen Arbor is in the middle of sleeping bear dunes. Not that many places to stay for a short visit in the summer and everything would most likely be for a week and pretty expensive. And Glen Arbor is wall to wall people and restaurants are packed.
When I think of Sleeping Bear, I think of Frankfort to Leland. Along M-22. Very scenic drive.
The hardest thing is finding a place to stay. Lots of great little towns to explore.
Me personally, I would go after Labor Day. Good weather, the crowds are down. But then I’m not a fan of crowds.