Seoul: 3 days (3 days will be sufficient) It is easy enough with the subway in the city, it is easy enough .It is enough to make amusement parks or North Korea border trip.
Gyeongju: 1 day (It is enough to see the oldest city of the country you are looking for and visit the structures for 1 day. If you want to go to other areas in the vicinity,
Busan: 2 days (Busan is a very big seaside city, ideal place for sea-sand-sun but when we went the weather was both cold and rainy, but the beaches are still worth seeing and there is also martyrdom in this city. you can add more days
