In southern California, La Costa is known the world over. Yes, it's exclusive and expensive. But you don't have to be an overnight guest to use the facilities. The new Carnival Destiny has a 15,000 square foot facility. The refurbished S.S. Norway has a luxurious new spa.
Many ships offer combination treatments at discounted prices, sometimes less than land-based spas.
And of course, you've already paid for your room and meals. I've saved the best for last. It's called Rio Caliente . It's nestled in the mountains just outside Guadalajara, Mexico. Instead of traditional aerobics classes, several hikes are offered each day along the steaming river, or through the forested hillsides.