I've lived in the Cincinnati area all my life, but it's hard for me to think as a tourist.
However here's what I'd do.
1) Kings Island. If you can, I'd go for two days. If you like thrill rides (I don't, but I'm chicken), the rides are more exciting than WDW. And yes, buy your tickets at Krogers. In the Cincinnati area, there's one every few blocks. One other thing...go on a weekday, not on a weekend. Due to its regional, rather than its national appeal, you get a lot of people who come down for the weekend. Plus the locals, who can't come during the work/school week.
2) Reds game. I think Fridays are usually fireworks night. And on many fireworks nights, there is an offer: buy one ticket at regular price, and you can get half price tickets for the rest of the family. Don't have the exact dates on those, but you can find out the the website
http://reds.com . These seats must be ordered before game day though.
3) The Newport Aquarium. Somewhat overrated and pricey. However, the shopping plaza that it's situated in ("Newport on the Levee) is cool.
4) The Cincinnati Zoo. I like the zoo. But it depends on how many zoos you've visited. I wouldn't consider it a must see. But it's a good zoo with a lot of cool exhibits. I disagree with whoever said the zoo was large. I've seen much larger zoos.

The zoo is in a middle of a very urban area, and has no room to expand.
5) Air Force Museum in Dayton. It's worth the hour (or less, depending on what part of Cincy you stay) to visit the museum. Aviation history crammed into the museum. And it's free.
6) Cincinnati Museum Center at Union Terminal. It used to be the railroad station. It's huge, and the outside is beautiful. Inside, it has the Museum of Natural History as well as an IMAX theatre. Worth a few hours
7) Eden Park/Krohn Conservatory/Art Museum. The conservatory and the museum are both in the park. Nice way to spend an afternoon. The park overlooks the river...beautiful scenery.
8) Cincinnati chili. Before you leave, you gotta have lunch at one of our chili joints. Our chili is unique, and honestly, it's the only chili I like. My favorite is Skyline Chili, but there is also Gold Star as well. There's eitehr a Skyline or Gold Star in every few blocks. There is also Camp Washington Chili, but there is only one location.
Have fun. We have a lot to offer.