From Webkinz Insider Wiki
The Spa is a vacation option available on Vacation Island. In the spa, your pets are pampered until they are at maximum happiness, health, and hunger levels. The cost to send your pets depends on how low the levels are, and which ones even are low. It is cheaper if you feed/cheer up your pets before you take them to the spa. While your pets are being pampered, you see a picture of a spa with Webkinz pets in it. You cannot actually see your pets, and there is nothing to do at the Spa but watch the screen. Taking your pets to the spa only takes a few seconds to do.
Because the Spa is on Vacation Island, it is for deluxe members only.
At the end of the spa treatment, the pet you sent to the spa will have 100% happiness, hunger, and health levels. Taking your pet to the spa is helpful when you have pets that aren't happy, healthy, or full.