I’ll second a couple of the suggestions upthread. The zoo is very nice, and the Museum Center at Union Station is very good. Downtown has some very expensive, very nice restaurants, and some, as Driver8 says, that are the sort of bland places you see in downtowns everywhere. My favorite restaurant downtown is Bistro JeanRo, which is moderately pricey but charming and wonderful. (Not to be confused with its sister restaurant, Jean-Robert at Pigall’s, which is Mobil Four-Star/AAA Four-Diamond and damned expensive).
If you cross the river into Newport, there is a pretty nice shopping and dining area called Newport on the Levee. I find it to be pretty much just a nice mall, but the walk along the river is pleasant, and the aquarium is wonderful. If anyone in your group has mobility issues, they might find walking through the aquarium to be daunting, and the admission is steep, but it’s a hell of an aquarium, especially for the Midwest.
One town over from Newport is Covington, which has a neat old main street. There are a couple of restaurants there that have decent Jazz most nights – Chez Nora is my favorite.
Back on the Ohio side of the river, just a few block from my house, is the oldest continually-operating observatoryin the Western Hemisphere. They have public viewings most Thursdays and Fridays. You have to call for reservations, but it’s only $5 a person. I spent Halloween looking at the bands on Jupiter and a ring nebula. The telescope has this glorious old steam age look, and you get a very sharp view. When we went, we saw Jupiter, then we had to stand around awhile and wait for the other guests to step up and look, then they swung the telescope around to find the nebula. So, there will be a fair bit of standing around waiting your turn. I thought it was worth it, but just a head’s up.
Eden Park is (I think) the biggest park in the city. It’s where the big art museum is. It also has the Krohn Conservatory, which is free. It’s a good way to spend an hour or two.
If you want to take in an art museum, consider The Taft, a small but excellent gallery in a beautiful old house near downtown.
There’s not a lot going on in Fountain Square right now, but if you’re going to be downtown, you should drop by and oogle the fountain.
Graeter’s ice cream shops are all over the place. If you have a sweet tooth at all, you must try their ice cream. When my wife’s current employer was trying to recruit her to move to Cincy, they shipped her a box of Graeter’s ice cream.
There is a neighborhood called Mt. Adams, which is at the top of a very high hill overlooking the city. Drive up there to gawk and some old houses and to get a great view of the city. There are also some good restaurants, either for lunch or dinner.
I hope this helps. Have a great time in our fair city!