You could catch an event at Seongdong Takgu Gyosil, a stadium in Seongsu-dong. Why not take a stroll along the riverfront and watch the sunset while you're here? It's also full of great shops and restaurants, so you can spend the afternoon having a wander. If you think Seongdong Takgu Gyosil is cool, you also might like Seoul Olympic Stadium and Jamsil Arena, both located nearby.