Summer monsoon (jangeuma) can last hours. Switch to museums, bathhouses and malls instead of cancelling the day.
Yongsan. Huge, free permanent halls, good café. Easy subway. Pair with Itaewon if the rain pauses.
All-day bathhouse + sleeping rooms. Bring a small towel; follow gender-separated wet areas. Not a swimsuit venue.
Gangnam/Jamsil malls with aquarium, bookstores, food courts. Classic family rainy-day.
Covered coffee culture. Many cafés close by 21:00; check Monday hours.
Nampo and Seomyeon have long underground arcades when the typhoon wind starts.
Indoor galleries at Gyeongbokgung if courtyards flood. Floors can be slippery — slow shoes.
Packing list umbrella/fan items live on the packing checklist. Forecasts: seasons map.