If you're flying the same airline on the same ticket, your airline might even put you up in a hotel for the night. I know Korean Air does. I'm sure Asiana does the same.

The limo bus is a guaranteed hour each way, sometimes more, depending on where you are going and traffic.

What you should do is take the AREX to Hongik University (Hongdae). There's DEFINITELY nightlife there. Shopping, cafes, restaurants, clubs, bars, artists, music and people at the playground, you name it.