Gwangjang Market is loud and packed but that is the fun of it. Go before noon if you want a seat, and bring cash because a few stalls still do not take cards.
Seongsu is my favourite walk in Seoul. Most roasteries have English menus and the staff were happy to explain the beans, and there are plenty of quiet seats on a weekday morning.
Myeongdong is crowded but the tax refund kiosks are easy to find and most shops accept cards. Come on a weekday evening if you hate queues.
Itaewon at night is the easiest place in Seoul to eat when you do not speak Korean. Most menus have English, and the halal places near the mosque were a lifesaver for my family.

Seongsu Cafe Street
城東区

Itaewon Street
龍山区

Gwangjang Market
鍾路区

Myeongdong Shopping Street
中区

Mangwon Market
麻浦区

Dongdaemun Design Plaza
中区

Garosu-gil
江南区

Gyeongbokgung Palace
鍾路区

Samcheong-dong Street
鍾路区

Namdaemun Market
中区

Yeonnam-dong Gyeongui Line Forest Park
麻浦区

Apgujeong Rodeo Street
江南区

The Hyundai Seoul
永登浦区

Lotte World Mall
松坡区

Hongdae Walking Street
麻浦区

Euljiro Nogari Alley
中区

Insa-dong Ssamziegil
鍾路区

Bukchon Hanok Village
鍾路区

Ikseon-dong Hanok Street
鍾路区

Cheongdam Luxury Street
江南区