Traveling around Seoul for a few days with just a Brompton seems to offer the best value for money

Jongno-gu, Jung-gu, Dongdaemun-gu, Yongsan-gu, Mapo-gu, Eunpyeong-gu
Seoul's old downtown areas near palaces and fortress walls
Taking it easy through the Gangbuk area
All you need is a little bit of money for motel fees and food