While you are in Jeju Island, you can consider visiting one of the orange farms there, and at the same time, get yourself some very nice chocolates.
On our way to Seongsan Ilchulbong, we actually stopped by one to have a look. At the time of our visit, most of the produce have been harvested, hence we get to see only those not-so-ripe oranges.
The orange farm which we visited also sell chocolates, available in a few varieties - Jeju Cactus Chocolate, Jeju Mandarin Chocolate and Jeju Hallabong Chocolate, to name a few.
As for me, I like Jeju Hallabong Chocolate the most. Hallabong is a seedless and very sweet citrus fruit, and it looks quite like an orange.