We visit friends in Cincy a couple times a year. We've done the aquarium and had wonderful food at a German restaurant (I can get the info from them if you are interested) over the last few visits but I think the place the kids have loved best so far is the
museum complex. It is in the Cincy train station and holds the children's, natural history, cincy and train museums all in one (I may be missing one).
You can get a combo ticket that gives you entry into all of them, or just one.
Our combined kids (2 5yo boys and a 9yo girl) really enjoyed it there.
Our friends live about 10min away from King's Island and we have yet to go there! We'll make it one of these trips!