Two categories blend
Hotel-spa-with-treatment-menu (€200–800/day) and historic-thermal-bath-public (€20–80/day) are distinct experiences. The cities below are best in either category — sometimes both.
Vienna
Vienna Hotel Sacher Spa, Park Hyatt Vienna Spa, The Ritz-Carlton Spa. Therme Wien suburban thermal complex. Strong contemporary spa scene complementing the traditional Kaffeehaus-Konditorei culture.
Geneva
Hotel d'Angleterre and Beau-Rivage Genève spas — Swiss-luxury-tier. La Réserve Genève spa with Cinq Mondes treatments. Lake-view hammams. €350–600 day passes.
London
London The Lanesborough Club & Spa, ESPA Life at Corinthia, The Spa at Mandarin Oriental Hyde Park, Aman Spa at Connaught. £150–500 day passes; treatments £250–600. Mayfair and Knightsbridge dense.
Budapest
Budapest Gellért Spa (Art Nouveau historic), Széchenyi Spa (largest, neo-Baroque), Rudas (Ottoman-era). €15–35 day passes — fundamentally different price tier from luxury-hotel spas. The thermal water is the star.
Baden-Baden
German thermal town — Friedrichsbad (1877, traditional Roman-Irish bath) and Caracalla Therme. €30–50 day passes. Combines with Brenners Park-Hotel Spa for luxury layer.
Reykjavik
Reykjavík Sky Lagoon and Blue Lagoon (45 min outside city). Volcanic landscape spa experience unique. €70–150.
Strategy
Hotel-spa day passes often half-price weekdays. Book treatments separately from access — many spas charge for treatments and pool access independently. Thermal wellness companion focuses on public-bath culture.