upcoming Japan trip

once and for all i'll be poking around a part of japan ... and this time with a friend for 4 nights and 5 days. here's the rough itinerary:

• from Seoul to Busan by train
• from Busan to Fukuoka by ferry (stay overnight in Fukuoka at a CouchSurfer's house perhaps)
• from Fukuoka to Kumamoto by train (they have an old Japanese Castle there; maybe stay overnight)
• from Kumamoto to Yufuin by bus/train (they have hot springs there; stay overnight at the hot springs place)
• from Yufuin to to Fukuoka or to Hiroshima on the last full day of the trip (stay overnight in Fukuoka) then depart the next day from Fukuoka and back home

i'd really like to visit Hiroshima but the train costs are quite expensive so that is pending further investigation