Taxi is quickest, and it will probably cost around 50,000 won (~45 USD) and take about 40 minutes, depending on traffic. If you want to take the scenic route, you can board the 306 bus from outside the arrivals hall at Terminal 1--it will take you to Dong Incheon Station, which is about a 52 minute ride across the northern bridge connecting the airport island to the mainland. At Dong Incheon Station, you are almost at the end of Line 1, which ends at Chinatown. Hop on the train for two stops, come out of Exit 1, and there you are! Chinatown is across the street. Total cost: ~2,500 won.

No, no KTX nearby, but you can take the Incheon 1 Metro Line from the terminus at Incheon (right across the street from the gates to Chinatown) and go to Seoul Station and catch the KTX there, It takes about 70-75 minutes.

I’m so sorry no one answered your question on time! But I will answer it just in case other people have the same question. You can take a cab to the airport, it will take around 30,000won. Or you can take the public bus number 307 from Chinatown to “서부공단입구”, and then transfer to bus number 111 and get off at Incheon Airport floor 3.

Bryan M is right. Venturing out to Seoul, which is 1 hour bus or taxi ride away, to spend your layover time is a bit difficult. There is a much better and safer way to utilize your layover time though. You can try "Incheon Airport Culture Tour Program" which is free guided tour led by English speaking guide for transit passengers. You can choose 1 hour, 2 hour, 3 hour, 4 hour, or 5 hour tour depending on the time of the day and how many hours you can spend for the tour. Just Google it by keying in keywords "ICN free transit tour", and clicking the search result "Free transit tour" which usually shows up on or near top. The site is Incheon Airport site. There, you can click "Free Transit Tour" tab, and look through the choices, and join the tour(s) of your choice. Details on reserving and joining these tours shown on the website. Since these guided tours are starting from the airport and ending at the airport you don't have to face uncertainties and anxieties explained by Bryan M. Enjoy your free cultural tour aboard a comfortable tour bus!

Yes, you can take "Incheon City Bus Tour". You can buy the ticket at Tourist Information Center or from the Driver. It's Hop On Hop Off Bus which run around Incheon (Chinatown - Wolmido - Songdo Areas). Will you stay over night at Incheon too? I would like to reccommend you to stay at "Gyeongwonjae Ambassador Hotel". It's very good hotel in Hanok Style (Korean Traditional House)... but have modern facitities. It's located in Songdo Area and not too far from Subway Station - Central Park (Incheon 1 Line). It's also connect with Central Park Songdo, Hanok Village Restaurant, Shopping Centers, etc.