Ganhyeon has some fun stuff, but when I think about multiple days off I think of going BIG!! There are so many better climbing areas around Korea!
Seoraksan is THE must-see natural wonder of Korea, without which your trip here is incomplete. The granite there is to die for! The long routes on Janggun-Bong are surprisingly user-friendly, only requiring one set of cams. There is also great sport climbing, with good bolts, sweet location, and really fun moves. The hiking there is world famous, and Biseondae shelter is nice for resting your weary bones.
Seonunsan is world class as well. This is THE place to push yourself, with amazing choice of routes. If we're a diverse group of climbers wanting to try different grades, then Seonunsan is perfect.
There are some great climbs at Ganhyeon. My main beef is that I've done all the good routes, except the 5.13s. Also, there is nothing if you're climbing between 5.11a and 11d. And the camping is better elsewhere.
Let's dream big my friends!