COEX is a large underground mall complex in Samseong-dong, Gangnam. Samseong Station (Line 2) and Bongeunsa Station (Line 9) both connect straight into the basement levels, so apart from the short crossing to the temple you barely need to go up to street level. That makes this a good choice on rainy days, in the peak of summer or winter, and when air quality is poor. Allow three to four hours depending on how long you shop.
1. Starfield Library (30-40 minutes). From Samseong Station, head toward Exits 5-6 and follow the signs for COEX; the underground passage leads you to the mall's central plaza. The library is an open atrium with enormous floor-to-ceiling bookshelves, free to enter, and anyone can sit and rest there. It is a major photo spot, so the stairs and railings get genuinely crowded on weekend afternoons. Weekday mornings are much calmer.
2. The mall itself (1 to 1.5 hours). Shops radiate out in every direction from the library: fashion, cosmetics, bookshops, electronics, and large food courts spread over the floors, with passages continuing to the exhibition halls, nearby hotels and the Hyundai Department Store attached to the complex. The layout is easy to get lost in, so use Starfield Library as your landmark and check a floor map once at the start. When a big convention is running in the exhibition halls, the corridors and restaurant queues get noticeably longer.
3. Bongeunsa Temple (40-60 minutes). Leave the mall on the north side and cross the main road; the temple is about a five-minute walk. If you would rather stay underground, Bongeunsa Station (Line 9) Exit 1 puts you closest to the temple entrance. A temple over a thousand years old, ringed by glass office towers, is the highlight of this loop. Entry is free, but cover your shoulders and knees, keep quiet inside the halls, and do not photograph people who are praying. Short temple-experience sessions for visitors are sometimes offered, so check the temple's official website in advance for the schedule and how to reserve.
Practical notes. Foreign-issued cards work in most larger stores, but smaller shops and food-court counters may take only domestic cards or cash, so carry a little cash. For the subway, buy a rechargeable transit card (such as T-money) at any convenience store. For VAT refunds you will need your passport: above a certain purchase amount some stores deduct the refund at the till, otherwise use a self-service refund kiosk in the mall or process it at the airport. The aquarium, cinema and ticketed exhibitions sell out on weekends, so book online. Coin lockers in the stations and inside the mall are useful if you are carrying luggage.