Okay, this post is late, very sorry. But it is not too late to check out this year's Seoul Fringe Festival, taking place right now in Hongdae.
This is the 11th Seoul Fringe Festival, a two-week celebration of independent, underground and sometimes just plain weird art, music, theater and other cultural goodness. Do not expect to see many of Hongdae's biggest bands at Fringe. The whole point of the show is to give unestablished artists a chance to show off. They might be raw and unready (and they might not be any good), but I think it is good to shaking things up from time to time... especially in a place as hierarchical as Korea.
The festival takes place pretty much all over the area around Hongik University, with the bulk of events happening along the "meat street" area (the twisting road/park that runs just behind the LG Palace, KFC, and Soundholic).