Jeju Island （済州島）
Jeju is popular for Korean people. It used be a honeymoon trip destination for Koreans before. I think now a Korean honeymooner goes abroad better than this island. It is much warmer than Korean Peninsula, their main lands.
There is a crater mountain on the island. It’s like a small Diamond Head in Hawaii.
Busan is very close to Tsushima of Japanese territory, also not far from Fukuoka. There are transportation services by ferrie between these cities. The city has old forts and their long walls surrounding the city.