When the Spa at Séc-he opened its doors in Palm Springs in 2023, it welcomed guests to a 73,000-square-foot facility with mineral-rich bathing waters, resort-style pools, salt caves, steam rooms and a slew of indulgent amenities, all without requiring booking a treatment. Of course, massages, facials and the like are available, but a major draw of visiting Séc-he is simply to spend the day cocooned within its grounds. The spa isn’t alone in selling day passes to visitors. North of Palm Springs in Desert Hot Springs, many hotels pump mineral waters from the ground into oasis-like enclaves where spa-goers can soak for the day without staying overnight. About This Guide Our journalists independently visited every spot recommended in this guide. We do not accept free meals or experiences. What should we check out next? Send ideas to guides@latimes.com. And further south in Rancho Mirage, Palm Desert and Indian Wells, some of the splashiest resorts in the valley offer day pass admission to deeply impressive spas. Think aromatherapy showers, menthol-infused dry saunas, zero-gravity lounge chairs and …