I agree with LapLap. The Seto Naikai isn't exactly a "tropical" paradise, but it is desolate and very beautiful, in a "Woman in the Dunes" way.
My favorite place there is this one:
http://www.naoshima-is.co.jp/english/
They have a Tadao Ando-designed museum and hotel, and it is mega-cool. By the way, I spent a couple of days there on
my honeymoon, so I'm biased.