Missed you by a couple of days...i'm flying out of ICN today to SF. I reckon you have at most 6 hrs. If you are ticketed as a transit stop in ICN (there will be a X next to your destination), you may have to pay departure tax.

Even with 6 hrs...you have at most 3 hrs to check out seoul. I suggest avoiding the shopping areas, as it can be quite a maze. Go to the GyeongBokgung Palace. If you still have time, you can wander further to Insadong where they sell tourist stuff. Check with the tourist info desk at airport. I am sure they can provide a better itineary than I can. FYI, I find difficulty communicating with the koreans in English.

Hope this helps.