I spent about 15 minutes in the Presidential Gallery before heading to R&D, and when I go back I will definitely take a second trip to the R&D Gallery. Definitely make sure you check out the Hawker Kestral (VSTOL predecessor to the Harrier), the YF-12A(forerunner to the SR-71), and the
Convair TIFS (My aircraft design prof. was a flight test engineer on this aircraft before it was retired) Many of the Volunteers are USAF retired, and love to talk about the various aircraft, I had a great conversation about the TIFS and SR-71 with a women who had been a Blackbird flight mechanic.
Allocate most of the remainder of your day to visiting the various galleries, my father and I spent an entire day walking around and talking about the various aircraft. As it was we didn't spend a whole lot of time in the Early Years Gallery. Make sure you bring a camera too, my phone battery was dead by the end of the day from taking so many pictures. I did the math and I took a picture once every 2 minutes. I recommend panorama mode as well.