Dear …
Thank you for hosting the __(name of retreat)__ retreat for __(name of organization/type of participants/etc.)__. It was an incredible experience for me and I am so thankful I __(was sponsored to be/had the privilege of being/etc.)__ able to attend this retreat. The environment you provided for us was quite unique and I really appreciate the time you gave to introduce us to __(subject matter)__. I was very intrigued to learn about __(briefly described the most profound thing you learned)__.
__(You/and your staff)__ did a commendable job, __(Name)__. It was both enjoyable and very informative. It was great to see how you captured the interest of the all the participants, as your expertise in the __(type of expertise)__ area shone through. I thoroughly enjoyed this retreat and will be expressing that to all those I know. This type of experience goes a long way in educating us in the area of __(type of area)__. There were many positive things I was able to take home with me.
Once again, thank you. I applaud you for your efforts in this field.
Sincerely,