I imagine renting a big AirBnB or a couple little ones and inviting people to hang out in Mill Valley. During the day we can work at the BnB or go to the cafes to write privately, collaborate on edits, go on walking meetings to discuss our blocks, take a break by strolling the cute town, or go on a hike.
The only problems are that the food is a bit expensive, and the cafes don't have very many outlets.
I would want to run a couple events during the retreat, some one-off, some ongoing:
First Night of the Retreat: 2 minute lightening talks by each author in which they describe what they are working on, why they are inspired to write it and what might be blocking them.
There is a weekly Tuesday Night Trivia at a cute event center in town. Being out in the world together as a team could facilitate a sense of community within the writing group. Its also fun to see your collaborators in a very different, less heady context.
Peer editing of essays - self organized
Walking meetings to discuss blocks or ideas - self organized
Challenge the author or brainstorm how to make an essay or an idea better - volunteer based
A morning hike in the woods
I would hope an event like this could catalyze more, an form a long lasting community of writers.