Offsite retreats seem like a bad idea in general, I've never been to one where I felt the team or company worked together better afterwards, and I've seen some spectacularly bad results... like the time the facilitator tried to get to the bottom of tensions between the CTO and head of sales (the classic problem of "Engineering isn't shipping features fast enough" and "Sales is selling features before even consulting…
* Do the bulk during work hours.
* Be 100% accepting of people who don't wanna spend free time on it. Or compensate.
* Add in loads of general fun.
* Let the employees be in charge of what constitutes fun.
* Go light on the actual work.
* Plan well ahead, respect people's own schedules.