I haven't gone to Jeju Island but I have heard from some of my friends that they have great beaches. You might have to consult their travel ministry as to the best time of the year to go to the island.
What will make that place really worth the visit is the fact that you are on unfamilliar territory and that you will be discovering a lot about the place, and you only have yourselves which in turn will add to the bond between you and your companions.
So whether you are going to South Korea, or Jeju or Boracay, do it now and capture those memories to last a lifetime! over 5 years ago