We often stopped for lunch in Cadillac when driving from Holland, MI to Mackinac Island. We always ate at Hermann's European Cafe in downtown Cadillac. The homemade soups are very good and I always enjoyed the fish dishes, especially the whitefish. The Chef's Deli side of the restaurant is also good for a quicker meal and we would always get something sweet from the deli to take on the road with us.
It's easy to find, right on the main street of Cadillac.