Start with your flight times and count nights separately from sightseeing days. Arrival, checkout and departure dates can each lose several hours to transport, baggage or waiting for a room.
Three or four nights: stay in one city
A short stay usually works best with one base and nearby plans for the first and last dates. Seoul offers more room to rearrange neighborhoods; Busan needs time for its chosen districts; Jeju asks you to settle the transport plan before choosing an area.
Five or six nights: add a city only for a reason
A second stop can fit if you have a clear reason to move and can still leave complete days in both places. Treat the KTX or flight as a transfer, include hotel checkout and check-in, and avoid a fixed reservation soon after arrival.
A week or longer: decide whether Jeju deserves its own block
Seoul plus Jeju uses a separate flight and island transfer, while Seoul plus Busan uses a rail journey. These are different trip shapes; choose the one that matches the places you want rather than trying to include both by default.
Before paying for hotels, mark the usable morning, afternoon and evening on each date. If a city has only a transfer evening and a rushed departure morning, add nights or remove that stop.
Plan the intercity leg
Korail schedules and tickets · Compare Seoul, Busan and Jeju for a first trip