Here are some exciting treatments to try. The Guerlain Spa at Raffles London at The OWO is one the largest—and best—hotel spas in London, stretching across four floors and 27,000 square feet.

The hotel is a favourite of celebrities including Chris Hemsworth and Julia Roberts, and the spa treatments are equally top-shelf. The 90-minute La Prairie Life Matrix Immersion facial ($700 ...

And since many spas incorporate botanicals sourced from the Sonoran Desert (like prickly pear and cactus flower) into their treatments, a visit to the spa can be as educational as it is restorative.