When I first visited Seoul, I stayed in Hongdae. It's a great central location if you want to be close to lots of things to do. As a first-timer, being right in the thick of the action in Hongdae was really fun. Lots of cool bars, live music, street food, and young locals give it a lively vibe. Gangnam is also nice if you prefer something more upscale and commercial. You can't go wrong with either of those, especially for five nights.