Zero Mile Art
Zero Mile Art is at 501 Whitehead Street, on the corner of Fleming, which is the corner where the green and white sign marking the end of U.S. Highway 1 stands. That sign is one of the two or three most photographed objects in Key West, and a steady stream of people arrive at it having just driven down the entire length of the Overseas Highway to do exactly that. The shop is right there, so the photograph and the browse happen in the same stop.
Inside is a mix of local artwork and prints alongside handcrafted gifts, souvenirs and Keys memorabilia, which makes it a reasonable place to solve the problem of what to bring home. It also sells key lime pie, snacks and cold drinks, which matters more than it sounds like on a Whitehead Street afternoon in August, and it is open from mid-morning until eight at night, so it is still there after the galleries down the street have closed.
The location puts you at the top of the Whitehead corridor, with the Green Parrot a few doors up and the Hemingway Home and the Key West Lighthouse a few blocks south, so this works as the start or the end of a walk rather than a detour.