Seoul Botanic Park combines an urban park, lake paths, and a large glass-domed greenhouse in the city's west. The greenhouse is the main reason to come, with plant collections arranged by climate zone and enough scale to feel like a proper destination rather than a neighbourhood conservatory.
The park is not central, but it works well if you are staying near Gimpo Airport or want a quieter green day away from the palace-and-market circuit. Give yourself time to walk the outdoor gardens as well as the greenhouse.