Something on the order of 1 or 2 weeks in length, super compressed that gives working devs a chance to learn something new or explore something outside of their normal job. I'd love to do something like this, particularly in an exotic location, but against a set amount of time that doesn't require me to quit my job.
We've adjusted the format of the current Hacker Retreat to accommodate people working (remotely, on their startup, etc) - 20% of your time is committed to learning, the rest of the time is up to you. The time-span is over 2 months minimum. We're currently taking applications for the Batch-02 starting in May/June.
Super-compressed, 1-2 week retreats are something we've thought about, especially for advanced topics like data science and functional programming...may do it later, but not this summer.
I love the hack camp idea and chance to fully dive into something with like minded people, it just needs to fit into something I can spend my vacation days on.