Boeing Factory Tour in Everett, WA just outside of Seattle is a definite must see in person when in the area. The sheer scale of the space and the planes and the factory in motion is mind blowing.
From the Boeing Site, "The Future of Flight Aviation Center & Boeing Tour is located in Mukilteo, Wash., 25 miles north of Seattle. Public tours of Boeing's Everett factory are available seven days a week. The Everett, Wash., facility is home to the 747, 767, 777 and 787 Dreamliner production lines. During the tour of the world's largest building by volume (472,000,000 cubic feet or 13,385,378 cubic meters), visitors will see airplanes being built for our worldwide base of airline customers."
Most fascinating is just feeling the SCALE of the factory and the amount of people and parts that go into the planes we fly on... it was stunning, and definitely worth seeing in person.