My wife and I love going to the Teahouse Spa. We usually get the Gemini package, which is time in the spa and a couple's massage. The massages are in two separate rooms, so after you soak in the spa, you put on a robe and slippers they provide and walk down the hall to meet your masseuse. It is a very fun time, and they give you a tea pot with different teas you can choose from.
The only downside is that the spa is located in downtown Santa Cruz, so if you are relaxing closer towards the evening (which I recommend, because the garden is lit by tealights), you will hear people and the surrounding nightlife.
As well, the first time my wife and I visited, we did a 7pm spa session, and we spotted two raccoons drinking from the garden ponds, and were worried that they were going to climb up into the spa with us.