You could go further south and spend a week in Kyushu. When I did it I hopped around a lot, but you could probably spend half the week based out of Nagasaki and half the week out of Kagoshima and take some day trips. I went to Fukuoka, Kumamoto, Nagasaki, Beppu, and Kagoshima in less than that, but wouldn't recommend that pace. Nagasaki is really special and oft ignored because it is so far south, but worth 2 days at a minimum.