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…
We have one annually for my company. It's at a casino, with a few scheduled meals and some activities (video game room, trivia done by a professional company). It's voluntary and employees are allowed to bring their spouse. I think it helps a lot with communication, especially since we are a distributed team and most peoplr don't see each other. But it's hard for me to know for sure, no one tells the owners the truth…
It's not free if the company is paying for it.