Koreans love hiking and in Seoul there are many opportunities to hike small mountains. There is even a mountain at Yonsei campus! I found lots of hiking meet-ups both on Facebook and meetup.com, much more than in Tokyo (although the meet-ups in Tokyo are more diverse I think).
One of those is a night hike meet-up organized by Warren Seung Il Kim who also does various other trips. We went to Achasan at 9:30 pm to hike to the top and enjoy the view from there. The hike wasn’t very hard and there was enough light to see the path. The view is great, but if you want to take good pictures you will need a tripod and a zoom lens. Unlike Namsan, Achasan is not in the center of Seoul so there is some distance between the mountain and the lights of the city.