The Air Force Museum is a great place to visit and it's absolutely free.  Of course there are gift shops and that can get expensive.  Starting at the entrance, they have a statue of Icarus and tell the story of him flying too close to the sun.  From there, you travel through the stories of balloons, gliders and the Wright brothers.  As you continue through you see the evolution of flight through WWI, WWII, Korea, Vietnam, etc, right up to the most advanced aircraft in the military inventory today.  They have details and artifacts of manned space flight as well as missiles.
   The tour is really worth the time and you can spend as little or as much time as you want there.  They also offer free tours with a tour guide leading you through the huge museum.  We spent about three hours there today, but Alex really wasn't interested in reading the history of everything.  If you're really into military aviation, you could probably spend days there.
    After our tour of the museum, we got some lunch and Alex got a haircut.  We also stopped and picked up some groceries since we need to turn in the rental car tonight.  Tomorrow we're just going to relax, then we'll head to Mammoth caves on Wednesday.