How about Okinawa? It's quite close. (I've never been)
Originally Posted by
N227UA
A few hours from ICN would include beaches in Australia, Thailand, Indonesia, Guam, or even Hawaii!
That's a very liberal definition of "a few hours". The shortest nonstop from ICN to Australia is 7700 km. Of those, Guam is the closest at 3200km.
I was in Jeju two months ago (Dec). Certainly not worth a long distance trip, but it's easy to do from GMP. The absurdly frequent flights and low full-Y prices (about 100kWon each way) make it easy. Depending on where you are in Seoul, it can even be a nice day trip.