Absolutely - I mean it's either sit in a hotel and do NOTHING or go explore a new city you've never been to before. Hongdae is the best place to do this - everything is open late, it's an ACTIVITY hub - meaning there's shopping and cafes and all kinds of food. There are clubs in one area, but you don't have to go there. And then easy check out the next AM at 11 and go back to the airport. Take the local AREX train.

You can't really see any tourist sites in central Seoul with that evening arrival and next morning return to the airport.