Most visitors to Seoul have spent time shopping, sleeping, or attending meetings/conventions at the COEX center in Samseong-dong, Gangnam-gu, Seoul. The COEX complex consists of a subway station, three hotels, casino, shopping mall, convention center, and countless restaurants. The shopping mall, which boasts of being the largest underground shopping mall in Asia, underwent construction in July and is expected to be completed by December 2014. A second subway station will also open with connections to Gimpo Airport. It’s also worth noting that the mall offers blazing fast free wifi. (Note: nearly all of the shopping mall is closed for construction as I discovered.)
Here’s a story on the overall future plan for COEX.