Nice quiz. Have been to all 6 of those islands. Maybe a good one to expand a bit, having said that most of the other islands are quite sparsely populated.
I suspect the difficulty with an expanded quiz would be the large number of densely populated reclaimed islands off the coast of Tokyo, Osaka and some of the other large cities. A lot of these islands do not form a single ward or district on their own and census information at the island level is probably impossible to come by. This map of Koto ward gives an idea of the problem: https://upload.wikimedia.org/wikipedia/commons/4/40/Koto_Tokyo_Map.png