You can make day visits to the Ludington State Park, of course. There's a nice walk (mile and a half, I think) to the lighthouse, for one thing. Not sure if there's a playground area (been a long time since I was there), but I wouldn't be surprised if there were one.
Take a day trip to Sleeping Bear Dunes. On the way there or back, if you want to swim there's a nice place, North Bar Lake, just north of Empire a couple of miles (take Bar Lake Rd.). Manistee has a good park with beach and lighthouse, too.
South of there, Silver Lake SP area has driveable sand dunes. You can rent a dune buggy and kick up some sand.