If you have enough time, a trip to Lotte World would be a great experience. Lotte world is like a knock off Disneyland, no seriously, the rides are comically very similar with even pieces in the rides that look like they were stolen from the Disneyland factories that make them. But it's amazing as HALF of the theme park is indoors, in a HUGE dome. It's the world's largest indoor theme park. And will only cost you about 25 bucks for a half day evening entrance. So you could go 5pm to 11pm. It's located on the other side of the river from all action, depending where you are in Seoul, probably a 15-20 min subway ride.