When I was looking at flights going to (and returning from) the Philippines, I knew I wanted to do a long layover somewhere. The question was, where. When I found out that Incheon International Airpot had free transit tours, that was it. I chose a flight that had a 15 hour layover, but our flight leaving the Philippines was delayed by 3 hours. Not a big deal though because we had plenty of time to get to the first transit tour. You have to reserve your spot on the tour in advance and sadly, the one that I wanted was completely full. Also, if you have enough time during your layover, you can book more than one which is what we did.
First stop is the transit tour desk to fill out a short form and then customs/immigration. They verify your name on the list and you go through customs and get a stamp in your passport.