This will probably make no sense to someone who is not intrigued by the concept of being in another's power. For me, however, the idea of being utterly helpless seems terribly relaxing.
My thought today is of being mummified. Wrapped tightly in gauze bandages from head to toe so that I am completely unable to move. Only a plastic tube in my mouth allows me to breathe and, of course, my captor can easily cover that with her fingertip or blow smoke into it. I am utterly helpless. Most people would probably view that helplessness as frightening, but I think it would be very relaxing for me. If I am completely helpless nothing is expected of me. I may die in the next five minutes but there is absolutely nothing I can do to prevent that... I can't even beg... Take that and combine it with complete sensory deprivation and I would be drifting in a world without time.
I can't think of a more relaxing way to spend a weekend.