We had a nice time at St. Joseph, MI a few years ago, beach house rentals are pretty easy to come by and we had a nice time.
A friend of ours parents owned an awesome place on one of the Kentucky lakes, an old funeral home turned into a bed and breakfast, or "Dead and Breakfast" as we referred to it, that had a big place, lots of room for several families w/ small squirrelly children. We had some good times there. I got into it w/ my wife recently going 'what the hell, of course you remember the water park in Kentucky' b/c I forgot she was on the DL w/ a brütal hangover....there's a lot of places down there though.
Another idea might be to head north. When I was a kid we went to Lake Nebagamon which is way up near Superior, WI, got cabins, drove boats around the lake. I recall driving my grandpa around in the boat while he swilled from a quart juice jar full of Manhattans. I can't wait to be a grandpa. The water is pretty darn chilly up there though. At my grandpa's funeral a couple of years ago, some of my mom's cousins stayed there and reported nice condos w/ hot tubs, etc.
Another possible trip would be the UP of Michigan. My wife's uncle owns a house up there, on land that was in his wife's family before the Mackinaw bridge was there. Good visit for a week, went to Whitefish point museum, saw the bell from the Edmund Fitzgerald, Sioux Ste Marie locks were interesting and also Mackinaw Island was an amusing day trip.