There are city tours specifically for people in transit at ICN. It's an easy way to see some of the major sites and not have to deal with getting yourself around, changing money, etc.
http://www.airport.kr/iiacms/pageWor...de=C1202020600
Most of those in transit will have a similar schedule, and the tours are based around these timelines. I did a city tour about a year ago and there were 8 of us in a mini-bus with a great guide.