5 Hours is way too short for something in Seoul as you will spend most of your time travelling between the airport and the city. Explore Incheon or as skchin suggested, there are a ton of things to do in the airport. Try some Korean food or even the Korean Baths (JimJilBang) in the airport if you want to relax in a cheap and relaxing environment. It's quite an experience. Plus you will get to scrub up nice and clean and feel like you are at a spa.