A beauty spa is much more than a simple beauty salon. While you can get all the services you can get at a typical salon, at a spa you get more: attentive, personalized service, special treatments like body wraps and mud baths, and even more exotic therapies imported from Europe and the Far East.
Given the level of service and the types of treatments you get at a beauty spa, you should expect a spa visit to be significantly more expensive than a salon visit. However, it's worth the extra cash, and you can always angle to get a spa gift certificate as a birthday or Christmas gift.
One great option that's usually not available at a beauty salon, but which you can usually get at a beauty spa, is a massage. A full-body massage is best for relaxation, but even a shoulder massage, a foot massage, or a chair massage can offer a great deal of comfort.
