Gangnam / COEX Mall

COEX Mall in the Samseong-dong area of Gangnam is Asia's largest underground shopping center. It's a massive complex with shops, restaurants, a cinema, an aquarium, and exhibition/convention spaces. The star attraction is Starfield Library (opened 2017) — two 13-meter-tall bookshelves flanking an open atrium. It's free to enter and perfect for photos. K-pop fans will find SM Town Museum and SM artist merchandise stores. COEX Aquarium is a solid 2-3 hour visit for families with kids. The basement food court offers decent portions for the price. Accessible from Gangnam or Samseong Station, it's a 5-minute walk to Bongeunsa (a 1,200-year-old temple in the middle of Gangnam), making for an interesting contrast between modern and traditional Seoul. Especially great as an indoor option on rainy days.