For those stressful days, get a massage at a stress free price!
Reflexology offers full body massages for $20-$30 dollars for 60-90 min.
Of course at that price, you won't have the ambiance of a Spa. However, you will be able to release some of the tension in your muscles and rejuvenate your mind, body and spirit.
My favorite in the Bay Area is Relax Feet in the Sunset on Taravel. They don't have a website, but I'd you Yelp them-you can find all there locations. They just opened one in the mission.