This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Tenaya Stone Spa is the Disneyland Resort onsite spa inside Disney’s Grand Californian Hotel & Spa offering body treatments, facials, massages and some wellness classes. At the time of this writing, Tenaya Stone Spa is the only spa inside any Disneyland Resort hotel, but its offerings are available to other guests.
For advice on how to choose a Disneyland Resort Hotel, see this article and for making the most of your stay, see this one.