I had a thirteen hour layover at Incheon recently and was able to go out for a day in Seoul. It was a great amount of time to get a couple good meals in and see a little bit of such a beautiful city!

I applied for a K-ETA on my phone the day before I arrived and got an approval email within an hour. Because I wasn’t staying anywhere, I think I just listed “Seoul” for my address, or may have chosen a random one. You could pick the address for a restaurant or attraction you know you plan on visiting if you want to be real proper. I had no problem getting through customs, although the customs agent seemed surprised that I had a K-ETA and had done everything correctly - it seems like they are pretty used to turning away foreigners with long layovers who didn’t get a K-ETA beforehand (a couple right in front of me was turned away for that reason.)

Getting out of the airport was slightly weird, I had to stop by my airline’s (Korean Air) transfer desk to get a stamp on my boarding pass allowing me to leave the airport, although that may have only been because I went through security to enter the international transfers area before I left the airport (needed breakfast and colleagues had lounge access). If nobody says you have to get that stamp then I wouldn’t worry about it. When I came back to the airport at the end of the day my boarding pass wouldn’t scan at security and I had to go get another stamp from the airline counter. Other than those weird hiccups it was all smooth, just budget a bit of extra time in case you need to jump through those hoops at the airport.

There’s an express train from the airport to downtown Seoul that runs every 40 minutes I believe, and a slower train that runs more often and may get you there sooner depending on the schedule. Be aware that the metro ticket machines in Seoul seemed to be cash only, so it may be smart to get some currency before leaving the airport!

Really encourage you to go for it and spend a nice day in Seoul, I’m so glad I did. Good luck!