That is what I was intentionally thinking with the opening post. Though of course it has to reasonable. A scenario like the zoo and safari park is one thing, but if someone said, say going to Tampa to visit Disney that would be a different story.
Boston is also a very good one. They have the Franklin Park Zoo, Stone Zoo, New England Aquarium, and Museum of Science (which is accredited by the AZA). The EcoTarium, Capron Park Zoo, and Roger Williams Park Zoo are also nearby (And maybe the Buttonwood Park Zoo)
I’ve been messing around with a map site, being inspired by this thread. I pick a Zoo and place a circle with a radius of 60 miles from the center. I tried Stone Zoo and got a pretty good collection of other zoos within the circle. But then I placed it over Southwicks Zoo in Mendon, Massachusetts...we end up with the following: Southwicks Zoo Stone Zoo Franklin Park Zoo Capron Park Zoo Buttonwood Park Zoo Roger Williams Park Zoo New England Aquarium Mystic Aquarium Lupa Zoo The Zoo in Forest Park The Ecotarium And the Woods Hole Science Aquarium (just outside the circle). You also end up with a couple farm/petting zoos, and three small city Parks and Rec Zoos (mostly farm animals, Fallow Deer, Emus, and Peacocks). Plus three zoos that have closed in the last 40 years of so...mercifully the New London Bates Park Zoo and Norwich’s Mohegan Park Zoo...and sadly....Benson’s Wild Animal Park. The Boston area is not bad for Zoo buffs. The zoos are smaller, the Aquariums world class, and Roger Williams and Southwicks each have pretty nice collections. You also end up with four Natural History and two Whaling Museums (of differing quality).
What about San Francisco? San Francisco Zoo Oakland Zoo California Academy of Sciences Aquarium of the Bay Six Flags Discovery Kingdom CuriOdyssey
While maybe not as prominent as some others listed, I think it presents a good case. I'm not sure I consider Six Flags to be San Francisco, they're across the bay and up. Their collection has been slowly spiraling downwards as it is.