It's definitely a 2 day visit - I've been twice. Once at 14 when I didn't appreciate it as much now and then again a few years back. I snapped at least 2 shots of every plane and drove my wife nuts. I'd like to get to the Smithsonian Air and Space Museum next. But I plan on going there in another year or two to Dayton.
One specific place of interest :
Outside they have a 2 story brick radio and air control tower from WW2 England that they dismantled brick by brick and shipped over to Dayton and reassembled, re-modeled inside to era standards. Very cool. The guide at the time was a WW2 veteran pilot and was very informative and nice.