All good suggestions from mikesaidyes. I might add a few more.

There are lots of affordable and good quality hotels in the city if you want a place to relax during your layover. I prefer to use hotels.com to check for decent places to stay. The reviews by other travelers will give you a good feel for the hotel. Myeongdong is an area you might consider because everything is very close by. There are many airport limo buses that stop at various hotels in the area. Taking the limo bus back to the airport is a good way to get in a short nap!