oh i am heading to seoul incheon first and staying in seoul for the first 7 days, (return flight and hotel booked), then only heading to busan and then come back to seoul for another 2 or 3 days and depart back to sg from incheon.
Up to you though coz to me, it will be quite rushy and need leave busan much earlier. And if wan do last min shopping in Seoul, need carry luggage around.

