I'll echo that an 8 hour layover is likely not enough time to make it to the DMZ. I did the train/metro rather than bus to town on my layover back in August because I prefer to ride trains generally. Total time was just over an hour and not subject to the whims of traffic. I like that.

Assuming that you're there during daylight hours I'd suggest visiting the fish market or walking through the Hanok village. With a rushed visit you could hit both but that would be quite tight, especially with immigration and transfer times.