What are some travel tips to know before going to Seoul?

I’m planning to go to Gangnam, Myeongdong, Dongdaemum and Hongdae next year. I’m not buying their outfits because I know I couldn’t fit into them, I’m a US size S fyi. I’m not into makeup and skincare, all I care about are street food, museums and sightseeing. As a foodie myself I’m willing to travel for food if it’s worth it. I refuse to eat Sannakji because it makes me sick to the stomach just thinking of it wiggling, urgh… Which tourist attractions are worth visiting and what are other free things to do?

What are some travel tips to know before going to Seoul?
Post Opinion