The Nelson Atkins Museum of Art in Kansas City has an exhibit of photographs the show what is left of the sites of various world's fairs.
I suppose that world's fairs went out of favor as frequent flyer miles became popular: Why go to a mock-up when you can so easily see the real thing. One of my first trips as a child was to the New York's World's Fair but I was too young to remember it. There are childhood pictures of me standing near the New York State building which was subsequently made famous in the movie "Men in Black" in which the evil bug tries to abscond from the flying saucers that sit atop that building. And, on the drive to JFK Airport, you can still see the huge globe that was the centerpiece of the world's fair from the highway. The Vatican exhibit brought over the Pieta as its centerpiece, and Pope Paul VI visited there on his one-day trip to address the United Nations. The stained glass window's from the Vatican exhibit are now to be found in St. Mary Top of Fort Hill church in Groton, Connecticut. They are still very beautiful. My family also went to the 1967 Expo in Montreal, and I mostly remember the monorail that took one to the islands in the middle of the St. Lawrence River where the expo was located. I returned to that site in 1976 to watch the rowing events during the Olympics.
The world's fairs produced wonder back then. Now they have left ruins. The haunting images shown at the link above indicate that a trek down to the Nelson Atkins museum would be very worthwhile.