6-hr free time in Incheon or Gangnam

Our arrival at ICN is 7AM and our check-in time in our hotel in Gangnam is at 3PM.. Minus the travel time from ICN to Gangnam, we have a 6-hour free time to roam around either Incheon or Gangnam.

What places can you recommend for us to go to in these said areas?

Thank you ^{^}