Bukhansan (Seoul) and Seoraksan are excellent granite trad, though a permit is needed for Seoraksan (easily attained). Buchae Bawi in Busan also has decent granite trad. Meureung Valley, near Donghae south of Seoraksan on the east coast has some awesome cracks (though maybe not granite).
As for biking-- Korea has an amazing bike tour program. You can buy a bike-passport for W4,000 (includes a map in Korean). The main routes are Incheon to Busan; the 4 rivers, around Jeju Island, and the east coast (starting near Seoraksan). You can collect stamps and earn stickers (and medals) and there are plenty of rest areas (bathrooms, water. . .), free compressed air stations (esp on Jeju), bike-tels (accomodations esp set up for bikers; there are also lots of camp grounds, hotels, motels and such along the routes). They are pretty well marked, and pass tourist attractions. Google 4 rivers bike routes or stop at one of the many stations along the paths for guidebooks or more info. The (Korean only) navigation app Naver Maps has the bike routes.
"If you can't do something well, you might as well learn to enjoy doing it poorly."
-- from a de-motivational poster, but I find it oddly liberating