I've recently started hiking in the evenings after work. What a great way to spend my time! A physical work-out and some wonderful fresh mountain air (well, as fresh as you can get in the middle of this massive city), away from the harsh heat of the daytime. Whether walking up through the pathways and landscaped park at Namsan, or the more natural forest trails at Umyeonsan or Bugaksan, a couple of hours spent hiking after work are much better relaxation than anything I might otherwise do in that time.
Unfortunately it takes a 30-50 minute subway ride to get to any good mountains from my place, as I'm not lucky enough to have any right on my doorstep. But still, to have so many beautiful mountain trails just a short subway ride away is a privelidge enough for me! It's great to walk through the trees in the dark, and there's always enough ambient light from the metropolis to ensure you can see where you're going. I love Seoul!