If you want something that will fill a whole day without killing the pocket book its hard to beat the National Museum of the USAF. Located an hour north just outside Dayton. I would call it free but for anyone in Cincinnati that's not totally true because you have to get there which means filling the car with gas. Its an inexpensive trip however.
Walking through the place from the start, you can start anywhere you wish, but if you follow the time line you can witness the evolution of the airplane. The planes keep getting bigger, and you can see exactly where new technology appeared. The first metal monocoque fuselage, then the first metal monocoque wing for example.
Before they started all that flying foolishness the Wright Brothers were building cool stuff like this.