Can't think of any big resorts until you get up to Traverse City Area.
Up there you've got:
The Homestead
Boyne
Or way up to Mackinac:
Grand Hotel
Only thing like a resort in the South Haven area is
here or various small hotels near St. Joe (on the bluff near the beach).
The Homestead is quite nice for the summer - beach, pools, river rafting/tubing, restaurants, shuttle between hotels, hiking nearby. Only been to Boyne in the winter. Both about 6 hours from Chicago.
Grand Hotel is nice but even further and a ferry ride.
I love St. Joe's beach. South Haven's North and South Beaches are nice too. Saugatuck is nice (on a river, just a bit north of South Haven - beach is nearby).