One of the busiest restaurant is Myeongdong Gyoza. This place is basically "queue-sit-order-pay-eat-and go". This is the quickest service I had ever seen. Once you pay, you get your order in less than a minute.
Their menu consists of only 4 items each cost eaxctly 7,000 Won. Kongguksu consists of noodles in a creamy cold soy-based soup. The second item is bibimguksu, cold noodles in a spicy sauce. Mandoo is basically gyoza/dumplings. Finally, we both order the kalguksu, wheat flour noodle with four pieces of dumplings top with beef in a chicken broth. Along with the main dish, you get rice and for sure, kimchi.
At Myeongdong, we found a KrispyKreme doughnuts, we used to love it, but it ran out of busy in Montreal. Here, you can see the whole process of making doughnuts in front of your eyes.This is about it for the first day in Seoul.