I would have to agree that adding Kyushu is quite a trek. Fukuoka itself has an absolutely fantastic food scene, and a few touristy things but Kyushu itself has most of the attractions and it takes awhile to get anywhere - with a car or training.
Kyushu was a huge highlight for us but can require a lot of patience because of the travel involved.
Okinawa would be incredible, looks amazing but have never been.
Another side quest is going up to Nagano. Very peaceful, low-key place, great food and they have the snow monkeys which the kids would like. A shinkansen goes up there, even a baller version! And would keep you in the JR East area.