My friend Christian and I decided to do something not listed in any of the guide books, indoor fishing. It was definitely one of the weirdest activities I've done in Seoul, but still a lot of fun.
Essentially it was a big room in the basement of a building with a pool in the center and about 30 chairs circling the pool. For 10,000 we got fishing poles, a net, bait ("fish rice"), coffee, Ramen noodles, and an hour of fishing time. Every time someone catches a fish you weigh it and the weight gets displayed on a big tv screen. The computer system also keeps track of how many fish you catch.
Christian won a tube of toothpaste because he caught a super small fish and I won toothpaste because I caught the most fish.
Indoor fishing was strange but totally worth it if you are tired of the main tourist attractions.
Location Take the metro to Seokchan station and get out at exit 3. Walk straight about 100 meters and the building will be on your right. Walk downstairs and the door is also on your right. It's open from 2pm to 2pm.