Spa and relaxation tourism is growing internationally, and Peru is no exception. Many travelers come to Latin America from the United States and Europe who are eager to spend a night or two at a nice spa, where a day of massages, Jacuzzis, mud baths and more costs a mere fraction of what it would at home.
The best spas in Peru are currently found in the most important tourism centers, such as Lima, Cusco and Nasca. More remote locations such as Iquitos do not have as much to offer as of yet.
Most spas in Peru offer a variety of massages, steam rooms, aromatherapy and Jacuzzis. Some offer yoga, specialized treatments, sound therapy, mud baths and more. Every spa has its own specialties, so check what services they feature before you go.  Generally, massages and other services can be ordered individually, or as part of a package. 
Some of Peru's better spas:
Cantayo Hotel spa and Resort, Nasca. The Cantayo Hotel, one of the most elegant in dusty Nasca, is expanding its spa services. Currently, it offers massages and yoga, and will feature steam rooms, pools and hydromassage facilities soon.
Kallpa Wasi Spa, Yucay, Sacred Valley. About an hour from Cusco, those looking to relax after spending a few days hiking the Inca trail or exploring Machu Picchu might want to head here. It’s part of the Sonesta Posada del Inca Hotel.
Stilos Spa, Cusco. This spa is not affiliated with any hotel: you can't spend the night. It does have a good reputation and a wide variety of massages and treatments.
Miraflores Park Hotel, Lima. Features the Zest spa, one of the best in Lima. It's expensive, but the hotel is run by the prestigious international Orient Express group, so you know you'll get top-notch service.