Grand Haven or Muskegon MI are nice places. Muskegon even has a pretty good amusement park with several roller coasters and a HUGE water park. Michigan Adventures is what it's called. And the boating in the area is really good. Lot's of ramps and the ports are not more than 12 miles apart at any point. So you can put in at one and boat to another for the day.
Or Traverse City. Lot's to do for kids there. Some good boating too as long as the bay isn't rough.