As soon as I arrived with KTX in Seoul, there has been absolute nowhere else to go but the spa.
찜질방 or jjimjilbang. You literally live inside there for the night, and you can't get out after that until checkout. There's no need for you to get out as they have everything inside. The food, the spa, the massage, the saunas, the hot tubs, the ice room with all different degree of temperatures.
Apparently, all these hot bath and sauna contains all the minerals. To enter and stay there is cheap. The ones that is making you spend your money, is the food, or some arcade games. Yup, they have it there too. haha.