Sleeping Bear Dunes are worth checking out. We also enjoyed the cherry festival in Traverse City, I think it occurs over July 4th. The Admiral and I also rented a canoe and paddled down a "river", that was more like a creek with overhanging trees. Near the end there was a sign, "Divorced Yet?"...and let's just say it was a VERY timely sign to help us crack a smile. I don't remember much else about the area.
If any of your kids are artistic, you can check out Interlochen Arts Academy while you are up there...went to college with a dancer who went there. She was extremely bright, so I assume it was a good school. Interesting story actually....she majored in Art History in college so what did she become? President of a large division of a global clothing mfg and retailer and is currently CEO of a different multi-billion dollar publicly traded company. Sometimes parents and guidance counselors are wrong!
Further south, I recall Grand Haven being nice. We used to sail over to Michigan City (ironically in Indiana), New Buffalo and St. Joseph on long weekends from Chicago but not a ton to recommend there other than the Warren Dunes area..at least not back in the 90s. I do remember getting up to Grand Haven once though and being pleasantly surprised.