Thermal-spa culture is a European specialty. Here's the honest sort by depth.
Tier 1 — destinations on their own
- Budapest: Széchenyi, Gellért, Rudas. The world capital of urban thermal baths.
- Reykjavík + Blue Lagoon: Geothermal pools at every neighborhood (Laugardalslaug, Sundhöllin) plus the famous Blue Lagoon 45 min away.
- Karlovy Vary, Czechia: Centuries-old spa town. Drink the springs, walk the colonnade.
- Baden-Baden, Germany: Caracalla Therme + Friedrichsbad — the classic German Kur-stadt.
Tier 2 — strong spa-plus-city
- Vienna: Therme Wien for thermal, Sacher cake afterward.
- Helsinki: Sauna culture is the bath culture. Kotiharjun and Löyly.
- Bath, UK: Roman Baths (museum) + Thermae Bath Spa (modern).
- Istanbul: Turkish hammams (Çemberlitaş, Kılıç Ali Paşa).
Tier 3 — surprising
- Ourense, Spain: Outdoor thermal pools, mostly free.
- Tabiano / Salsomaggiore, Italy: Lesser-known Italian spa towns.
- Bath, Hungary (Hévíz): Thermal lake — Europe's largest naturally-warm lake.
Strategy
Pick a thermal city for 3-4 nights to develop the routine — bath in morning, walk and eat in afternoon, bath again before dinner. The day-trip approach to thermal cities undersells the experience.