I had a 7-hour layover in ICN last year. (connection between two different carriers) I took the Korean Air bus into Seoul. Very nice bus with big comfy seats.(two on the left and a single seat on the right) The bus goes into downtown Seoul area, near the City Hall. From there, I took a taxi and told the driver to take me to "all the usual tourist sites" within 3 hours. He spoke decent English and I saw plenty of Seoul(as much as one can see in 3 hours). At each location, I got out and took pictures and looked around.(managed to use up the entire 2GB CF card in my camera in 3 hours) I paid the driver the equivlant of USD$100 and he seemed very happy about it. At the end, he drove me to The Radisson Plaza Hotel where I took the Korean Air Bus back to ICN.

When I got back to ICN, I still had plenty of time left so I walked around the airport shops & restaurants. (had some sort Korean noodles... very tasty, but I didn't know what it was.. just pointed to the fake plastic food display and ordered)