Jeju is an Island in the south of South Korea in the Yellow sea. This island is well known by Korean for Tangerine, chocolate and honeymoon. A lot of Korean are going there for their honeymoon, especially in Jungmun where there are only Resort and beaches.
However, the other part of the island are really great and different.
If you like hiking, go up the Hallasan Mountain, if you prefer beach there are plenty of black sand in Jeju or one of white sand in Udo (20 minutes boat from Seongsan.
There are also plenty others activities as the folk village, the lava tunnel or some waterfall.
This island is excellent to relax after a trip in the busy Seoul.