King’s Island isn’t too far from Cincinnati, is it? It’s a roller coaster park.
Yes, if you like aviation museums at all, this one is not to be missed. Just make sure to give yourself plenty of time; I would say you need two or three hours at the very least. (If you like to take pictures of planes, allow twice as much time!)
Ah, it’s a shame my dad’s not coming, blondebear. The Air Force / aviation angle would be right up his alley.
Lots of good suggestions in this thread! We’re staying on the Newport / Kentucky side, so a stroll across the Purple People Eater Bridge is in order.
And awww, darn, the scene in the Avengers was, indeed, shot in Cleveland on Public Square in front of the entrance to Tower City. Or so says Tom Hiddleston in an interview in the The Plain Dealer. So close and yet so far! 
I will definitely try to talk DH into Jungle Jim’s. He was thinking maybe a Reds game so we’ll see if we get to go.
Half Price Books, duly noted. Anyone know if The Booksellers on Fountain Square is worth a visit?
Oh, and Bayard, thank you for the ice cream suggestions! I had heard of Graeter’s but never Aglamesis Brothers. Hoping to hit at least one of those. 
The Sign Museum. I freaking loved it, as did the guys I went with. Signs are really an art form - if you like Americana, you’ll be in heaven. So much neon.
Seriously. Wonderful.