Be fore warned, I think you might be pushing it with only 5 hours. Traffic can take anywhere from 1/2 hour to 2 hours from Kimpo to downtown.

I was flying from Seoul to Pusan and it took me 2 hours by subway to get to the airport, 1 hour in the air from sel to pus then 2 hours from the Pusan airport to the beach!!!

Now that being said, if it was me I'd venture out for a quick trip as sitting in Kimpo for 5 hours will likely seem like an eternity.

There are plenty of things to do on a Sunday in Seoul, what are you interested in? Shopping, sightseeing, eating out?? Let me know and I can give you some tips on places to go that are somewhat close to the airport.

The closest thing to the airport that might be of interest would be Costco/Priceclub, if you have a US card it will get you in and it's only 20-25 min from the airport! (12 subway stops on the same line as the airport)