The phrase came into common parlance when the 19c travel company founder, Thomas Cook, began taking travellers on Grand Tours of Europe i.e. they were taking a Cook's Tour. They would see all the "important" sites in the city they were visiting at the time and then move on to the next.
In modern terms, if someone does a "Cook's Tour", they whizz around all the well-known sites at their destination, without exploring any particular site in depth.
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