We've spent time at both Glen Lake and Crystal Lake, neither of which would be quite as far of a drive as Traverse City. They are lovely lakes for boating and swimming (chilly, but not so cold as Lake Michigan, I think). Both locations are accessible to Sleeping Bear Dunes, which is beautiful (
https://www.nps.gov/slbe/index.htm). Frankfort is a charming little town, and Point Betsie Lighthouse there is a pretty place to visit for a rock-collecting expedition.