You can drink with your new friend who you met during the first flight.
You can hit on a guy and make him buy you a drink. (Ah, yeah, you can do that anytime.)
If there's no one looks fun, you can get a massage or mani-pedi, take some photos, have some local food of somewhere not your destination saying "I'm going to LA and why am I eating a kimchi here?".
I had no idea Incheon Airport was this great. It's heaven.
There's free sleeping couches, free shower and free high speed wifi (seriously, it's so comfortable). We, nomad ppl, could live here.
Massage was not so cheap, though. As a guy leaving Korea says, it's the best way to spend the rest of Korean Won.