Sleeping Bear Dunes Nat'l Lakeshore in Michigan would be an awesome trip in that timeframe. The whole area around Traverse City and over to Lake Michigan is beautiful with lots to do and relaxing on the beach. From the park you can take a lazy canoe trip down the shallow Platte River and end up at a gorgeous beach on Lake Michigan.
You could drive up to the Mackinac Bridge and take the day ferry over to Mackinac Island. If the kids are old enough, rent bikes and tour the island (no cars allowed so no traffic to worry about - just horses).
This whole area is VERY popular so reservations are a must ASAP.