Having grown up in Bay City but now residing in Grand Rapids, I still think that "North" is anywhere above where 27 and 75 meet. My favorites are Traverse City and of course the Keewenaw. MTU for the win!
From near the southern border, up north is probably north of Mt. Pleasant. Cadillac for sure. My favorite place is the GH Day campground at SBD. Love that the beach is relatively secluded and not quiet and I can light a fire and take a bottle of wine to watch the sunset with my wife while the kids play in the lake. It's a local paradise.
Beaver Island and hiking on the Manatou's are phenomenal choices also.
Grew up in Garden City and spent almost every summer of my childhood in Rogers City at my grandparents and camping at Black Lake. Ahhhhhhhhh the memories.