Korea · island
The island is not a checkbox.
Hallasan sits in the weather like a rumor. Fields go green-black. Wind has opinions. Jeju is beautiful and slightly stubborn. That is why people fall for it — and why a rushed day-trip feels like insulting a mountain.
The rule
Add Jeju when the calendar still has air. Skip it when you are already late for your own itinerary. The island will not shrink to fit a guilt trip.
Two nights is a conversation. A same-day hop is a postcard you paid for in tired.
Otherwise the spine stays honest: Seoul then Busan. You can always come back when you mean it.