I haven't taken the free transit tour offered by the national tourism office or ICN. I heard that they are pretty good and will help you spend the 8 hours more fruitfully.

BTW, as stated by others, the Duty Free Shops in ICN are amazing. If only I had the money or the inclination. I can't remember if this is still so, but you can't buy booze to carry on US-bound flights -- US regulations on liquids.