The Andong Mask Festival, in Kyungsan Province, is pretty famous, and if you need to get out of Seoul, wouldn't be a bad idea.
Korea has a long tradition of Mask Dancing, and you can see some of it, and hear all the old traditional instruments.
And get out of the city, eh?
If you're not Korean, you can even set your goal to be this guy:
And possibly be featured on Arirang TV or Chosun Online (who love following white people around with cameras).
More information here.