Zaha Museum is the highest art gallery in Seoul, a hidden spot located in Buamdong (the location where Korean Drama Coffee Prince was shot) nearby Inwang mountain. You'll need to walk up the hill to reach this museum (somewhat tiring) but, you'll realize that it's worth the walk as soon as you arrive at the museum, breath taken by the beautiful natural scenery of the mountains.
Founded personally by Korean entrepreneur Kang Jong Kwon, the two story building Zaha Museum mainly exhibits experimental works of young artists. You can also enjoy the nice view of Seoul's mountain at the small garden on the second floor.