Last time I was in ICN, I had an 8 hour layover. Since coming from PEK, I wasn't interested in sleeping so I wandered around the terminal. I can't remember where it is but there was a kiosk offering transit tours. You tell them how long you have and they put together a tour for you. I think it was around $60 USD per person and they took us to Changdeok Palace, a temple and a market. They even feed you lunch at a restaurant in the market so you don't have to eat KFC at the airport.