So I spent nine days on a beautiful ranch in Boulder, Utah (yes, UTAH, not Colorado). It was wonderfully isolated, just us writers, the mice and the seven o'clock deer. And I was very productive, at least by my standards, writing over 200 pages.
I was productive, and I think it's important to understand this, not because the place was inspiring, but because it enabled me to be free of distractions. My wife very kindly handled all children and house issues for a week and half, so I was able to sweep the decks, not worry about stuff, and just write. I was up at 6:00 or 7:00 am every day, and spent all day, every day, sitting on the same couch by the south-facing window, writing until midnight or 1:00 in the morning (less time for hot tub breaks, hiking, food preparation and playing guitar on the porch).
In other words, you don't go on a writing retreat because you're stuck and you're hoping good ideas will come to you. You go on a writing retreat when you're not stuck, to buckle down, take no prisoners and crank out the pages.