Hongdae is the only answer, really. Can take the local AREX train easily, no change of subway lines. Land 9:45 on the train by 11, arrive in Hongdae by 12, leave by 5 PM, back to airport at 6 for 8 PM flight. Check out not only Hongdae area itself, but Yeonnam-dong and Yeonhui-dong TONS of cafes and restaurants and shopping.

You could also technically go into the center of Seoul for more traditional sights like palaces, markets, etc. BUT that walking and moving around between sites will eat up your time.