Day spa in or near San Francisco?

Hi!

I’m a young-20s guy who needs to find some sort of day spa in or near San Francisco to get away to this weekend.

Criteria:
* Has to be reasonably inexpensive
* Should ideally be a one-day thing, mostly for price reasons
* Has to be relaxing, and has to take me away from things. I’m having a horrible week that I’m expecting to get worse.

I guess that’s about it. Thanks!