Earlier remarks re legal prohibitions are very relevant
Jeju - both times I was there it was too early in the morning for photos.
The airport is very small and anything civil there may only be only be viewable / photographable from inside the small terminal or very close to it, no recollection of any good viewing sites there. The military side was too far away from here to identify anything other than the aircraft type and many aircraft were hull down behind sloping ground.
Kimpo - In theory a wonderful compact location for photography, compared to sprawling Incheon, but while a considerable number of Korean schoolgirls were using cameraphones to take photos from behind the glass near the gate, anything more organised or any sophisticated photography is likely to bring the security presence down on you mighty fast.