I've gone through Incheon several times - as others have noted it's a nice, modern airport with lots of duty free shopping and a couple transit hotels.
You didn't mention the airline, but if it's Korean the business class lounge is quite nice - with showers, food, drinks, and internet (but no proper sleeping facilities - if sleep is a priority check out the transit hotels:
Incheon(ICN) International Airport - Hotels ).
Here are some local tour possibilities if you're flying Korean:
https://www.koreanair.com/content/ko...ransit-program
Note that Seoul is about 30 miles away, so you'll spend a solid hour just getting there and back.
Personally, I've spent time in Seoul, so for a six hour layover I'd probably just hang out in the Korean lounge - for a longer layover I'd book the transit hotel.