Dongmun Market - where you can find fresh (and live) seafood and other produce. Some stalls have restaurants at the back and you can purchase the seafood and get them cooked for you. If marketing is not for you, a nice shopping street 七星路 and underground mall are within walking distance from Dongmun. 七星路 has many great international brands and was very quiet when we were there. No expectations for the underground mall though, unless you wanna get facial products or is an aspiring ahjumma. Dongmun Market is easily accessible from many places. Check out the buses from where you're staying.