Korea - 20 Hours in Seoul - What Would You Do?

I've had a schedule change, and one of my options is to take a 20 hour layover in Seoul, using ICN as a connecting point from Beijing to Tokyo PEK-ICN-NRT-SFO. I would land on Tuesday in ICN at 3pm, take off the next day at 11am.

Get an Incheon - Jeju - Incheon connection with a hotel package and enjoy walking on the beaches/ a round of golf/sightseeing and a great Korean sashimi dinner. Just check your early flight back arrives at ICN and not GMP.