Hey man check out the threads in this link.
A lotta good info there, and if you get onto isipau's links there are some top maps. Seoraksan/ Seolaksan/ Soraksan (many different transliterations) is a sweet trekking place. Closer to you might be Jirisan/ Chirisan which has good hike routes that can take 4-5 days. Both are great and different in style.
Damn! I wish I had 11 days off in May!