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
Seongdong-gu

Itaewon Street
Yongsan-gu

Gwangjang Market
Jongno-gu

Myeongdong Shopping Street
Jung-gu

Mangwon Market
Mapo-gu

Dongdaemun Design Plaza
Jung-gu

Garosu-gil
Gangnam-gu

Gyeongbokgung Palace
Jongno-gu

Samcheong-dong Street
Jongno-gu

Namdaemun Market
Jung-gu

Yeonnam-dong Gyeongui Line Forest Park
Mapo-gu

Apgujeong Rodeo Street
Gangnam-gu

The Hyundai Seoul
Yeongdeungpo-gu

Lotte World Mall
Songpa-gu

Hongdae Walking Street
Mapo-gu

Euljiro Nogari Alley
Jung-gu

Insa-dong Ssamziegil
Jongno-gu

Bukchon Hanok Village
Jongno-gu

Ikseon-dong Hanok Street
Jongno-gu

Cheongdam Luxury Street
Gangnam-gu