It's much more convenient to use the subway than a car for Seongsu-dong. For parking, there are some parking lots near Seoul Forest, so keep that in mind. Seongsu-dong's streets are long horizontally, so if you don't plan your route well, you might just end up with sore legs. I recommend a course that starts at Geondae-ipgu, then moves to Yeonmujang-gil, Seoul Forest Backstreet, and finally Seoul Forest, in that order, like I did. Seongsu-dong is a mecca for pop-up stores, so if you search beforehand to check which pop-up stores are open before your visit, you'll have an even richer experience.