K-pop instrumentals of course is another interesting part of K-pop. Just personally have not much time for listening.
Also many talented composers (most of them Korean, also some are foreign).
For example Super Changddai,
MonoTree,
OREO,
E. One, Crada, Hong Ji-sang.
Some mashups (combine acapella from one song with
instrumental from other song) can be also interesting,
and show some new details of original songs, not highlighted before, or give new experience.