Asan Spavis is a hot-spring waterpark with swimsuit-required indoor and outdoor pools, plus sauna zones, easy by car from Seoul, best on weekdays year-round.

Busan Hurshimchung in Dongnae is a huge hot-spring bathhouse with nude, gender-separated pools plus jjimjilbang rooms, easy by subway. Ticketed, busy on weekends.

Seokmodo Mineral Hot Spring on Ganghwa’s west coast has seasonal open-air baths and sea views, about 2 hours from Seoul, easy drive-up access, closed Tuesdays.

Paradise Spa Dogo near Asan is a swimsuit-required hot spring water park with indoor and outdoor pools, easy drive-up access, busiest on weekends.

Urban Yuseong in Daejeon has free 40°C foot baths plus nearby hot spring bathhouses, easy metro access, open year-round, no hike, bring a small towel.