i was only there for 1 day...
The DMZ tour is a bit boring but I guess you dont have much choice. I arrived very early as well however border control line was 1.5 hours long (depends if a Chinese flight arrives same time as you) by the time I got thru I had to rush and catch a train to get to the tour. The express was actually a 30 min wait so took the regular one. (you can purchase tickets at the automatic booth). I just did the JSA (was excellent) then on the way back the tour bus dropped us off by shopping. it is blocks and blocks packed with people and junk stores. After going thru that for a few hours a few blocks further after the junk shops you can find some better stores such as nice clothing stores for the misses and kids at pretty affordable prices and good quality (you have to weed thru the junk). We then took a cab to the Hyatt Seoul and they printed us directions to the chabad guy/Rabbi. the directions where off by a few blocks as he moved and had a very very hard time finding it (we walked form the hotel (approx 10 min down hill) to his house where his wife prepared us a delicious meal, just let them know you are coming. They are
extremely nice and accommodating (make sure to leave a nice tip). He also has a full stocked grocery in his house. We caught a direct bus back to ICN from the Hyatt (you can purchase tickets at the desk) for our early morning flight.
Are you flying Korean F? I was super impressed with them. The seats were not as comfortable as CX but the service was way better and wasn't sure why the CX rave on here compared to Korean.
The Lounge didn't have much kosher but had some excellent Hagen Dazs, and we took a few. (when the cleaning staff came by, she saw all the ice cream cups and she "said...Oh you forgot one favor you must try it") (Not Cholov yisoroel....., but I guess we were hungry

The also have al the Israeli newspapers (I guess allot of Israelis).
.. If you are flying F make sure to go upstairs to business and at the back of the plane (don't get fooled by the self service mini bar they have downstairs with big bottles of blue label, etc) the have a bar with couches (benches)where they will make you any drink cocktail you want (kosher?). They will have someone walk you and give some serious royal treatment and they will "inform" the staff all along the way .....'they are first class" and practically fight who can take care of you.
First was actually almost full with the entire airplane "Jewish" and we actually enjoyed schmoozing with some of the "Businessmen" and they had got a kick of the fact that we where all Jewish....
in general i found the Koreans although a bit rigid and "in their own world" type of people, they are one of the nicest helpful and honest people I have ever met thru out my travels. Prices aren't cheap and you can't really bargain or haggle that much or at all in many stores you can leave a $100 bill on the floor and nobody would touch it.
How this helps....