If you want Cedar Point, I would go with what the previous poster said. That's where I would go anyway, but then I grew up around Kings Island and live there now, so that would not be a vacation location for me.
Now, if you WANT to do Kings Island, there is a Great Wolf Lodge on site. It looks very nice (I have no reason to stay there, and have never been inside). You can do Kings Island in a day and be done with it, or you can make it a long weekend, as they do have a nice water park as well. If I WAS going to do Cincinnati as a vacation, I would do KI, then the Cincinnati Zoo and Botanical Gardens, the aquarium, the underground railroad museum, maybe catch a reds game (perhaps the Dodgers would be in town?), or take a riverboat cruise up the Ohio River. Also there are some GREAT restaurants in Cincinnati. I would stay downtown, and stay at the Garfield Inn and Suites. You can find, reasonably priced, one and two bedroom apartment style hotel rooms, complete with full kitchen, dining area, living room, 1 1/2 to 2 bath, and some have rooftop garden patios where you get a panoramic view of downtown.
If you like zoos, some of the best in the country are right here in Ohio. Cincinnati has always been rated one of the best zoos in the country, but Columbus has the distinction of being the #1 rated zoo in the country (thanks, Jack Hanna!) Cleveland is a pretty nice zoo too. The great thing about vacationing in Ohio, if you sit in the middle of Ohio, you can get to the following cities within 3-5 hours: Pittsburgh, Columbus, Cleveland (Rock and Roll Hall of Fame Museum) , Cincinnati, Indianapolis (Indy Motor Speedway, zoo aquarium, awesome children's museum), Lexington (KY Horseparks), Louisville, Knoxville, Nashville, Detroit (and then into Canada), North to Lake Erie, or South to the Smoky Mountains. I'm sure I am leaving alot out, but you get the picture. There is a lot to see and do.
