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
城東區

Itaewon Street
龍山區

Gwangjang Market
鍾路區

Myeongdong Shopping Street
中區

Mangwon Market
麻浦區

Dongdaemun Design Plaza
中區

Garosu-gil
江南區

Gyeongbokgung Palace
鍾路區

Samcheong-dong Street
鍾路區

Namdaemun Market
中區

Yeonnam-dong Gyeongui Line Forest Park
麻浦區

Apgujeong Rodeo Street
江南區

The Hyundai Seoul
永登浦區

Lotte World Mall
松坡區

Hongdae Walking Street
麻浦區

Euljiro Nogari Alley
中區

Insa-dong Ssamziegil
鍾路區

Bukchon Hanok Village
鍾路區

Ikseon-dong Hanok Street
鍾路區

Cheongdam Luxury Street
江南區