Spas with reservation required

Seattle, Washington

A private floating bathhouse on Seattle's Lake Union, booked out entirely for one group of up to 12 for sauna, cold plunge and hot tub rounds.

Palm Springs, California

Moroccan-themed inn in Desert Hot Springs with natural mineral hot springs, a spring-fed pool and day-pass access from 9am to 4pm.

Oslo

Waterfront sauna spot on Aker Brygge with shared and private saunas, sea bathing, and sauna boat trips on the fjord.

Bursa

A historic thermal bathhouse and small hotel in Bursa's Kükürtlü district, with separate men's and women's hamams, day passes, and overnight stays.

Santa Fe, New Mexico

A 77-acre hot springs resort outside Santa Fe with spring-fed thermal pools, day-pass soaking packages from $175, and overnight lodging.

Saturnia, Tuscany

Walk-in thermal park in the Maremma countryside with four outdoor sulfur pools, hydromassage jets and a vascular circuit fed by a 3,000-year-old spring.

Shibu Onsen, Nagano

Reservation-only private hot spring and sauna suites at a Nagano brewery complex, built around a craft beer bathing plan for two to four guests.

Cotswolds, England

A woodland sauna and ice bath venue near Northleach in the Cotswolds, offering communal and private heat-and-cold sessions plus occasional workshops.

Port Douglas, Queensland

Ayurvedic-style massage and bathhouse studio in Port Douglas offering couples treatments, yoga and reformer Pilates, bookings required with no walk-ins.

Rogaška Slatina, Savinjska

Wellness center at Grand Hotel Rogaška in Rogaška Slatina, Slovenia, with pools, saunas and fitness; day pass available by reservation.

Glenwood Springs, Colorado

A natural geothermal vapor cave in Glenwood Springs, Colorado, where visitors steam in ancient rock chambers and add massages, mineral baths or body treatments.

Wulai, New Taipei

A hilltop Wulai B&B with sodium bicarbonate hot-spring baths in every room and an outdoor infinity pool, geared toward couples.

Beitou, Taipei

A hot-spring resort in Beitou, Taipei with private suites, outdoor baths and overnight stays, open to day-pass visitors by reservation.