Security:
Try this now: Back when the 747 first came out, you could just walk down to the gate and check it out as part of their PR.
I remember that my buddies and I walked onto a 747, went through the whole plane (including an elevator that went to a lower area); I have a photo of me sitting in the pilot's seat. There was NO ONE there!
Someone went out and brought some beer aboard and we sat in the top level (back when it was a First Class lounge) and someone played the electric piano. Again, no one came along to move us out...the plane was there overnight for service or something.