One of my new favorite places is Longbread - a sandwich/salad shop that actually has greens. Salads in Korea haven't caught on too much yet, so ordering a "salad" usually means a piece of lettuce with some sort of assorted mayonnaise-drenched side dish on top. But at Longbread, they give you a generous helping of assorted greens and they have AMAZING toppings (think chicken salad, sauteed mushrooms, or delicious ricotta).
If you're not a salad person, they have some great sandwich choices as well - chicken salad or mushroom and cheese are some of my favorites.
So far I've been to two Longbread locations that I can highly recommend -
1) Gangnam Longbread: Head to GT Tower in Gangnam (exit 9) and take the elevator down to the basement. Longbread and the Flying Pan are right there next to the courtyard! (Haven't tried the Flying Pan yet but I've heard that they have wonderful pancakes!)
2) Myungdong Longbread: Get out of Euljiro 1 Station, and head to Lotte Young Plaza. Longbread is located on the first floor.