Stayed at the Holiday Inn Express in Hongdae at the start of October. You are maybe 30 mins or so from Gyoengbokung Palace and some of the more touristy areas using either the bus or subway system.
There is a direct bus from Incheon airport to Hongdae that takes around 1hr and drops off near the Apple store. You can also take the train from Incheon to Hongdae - although this is the local train that stops at all stations on the line. The line from Incheon to Hongdae also stops at Gimpo airport.
Plenty of shops, restaurants and bars in the area.