Maybe Samasati - http://www.samasati.com/ Nice place, dont know how intense a detox you are looking for but thats a start.
There is also Pura vida Retreat in Alajuela that gets very good reviews tripadvisor.com/Hotel_Review-g309224-d308790… and there is a new place that I am trying to remember the name but I just cant. Its at the tip of my tongue... ugrrrr..... I will give it more thinking and come back to you later. Pura vida does a lot of yoga, but also wellness in general and nutrition.
I just got word of this one too http://www.haciendadelsolcr.com/
Just remembered the other one, here's the link http://www.asclepioscr.com/
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