We do this at my company and it's awesome. The idea is it's not at all mandatory. Do whatever you want for lunch. But everyday, at noon, there will be a free lunch served at this location. Most people will participate in that 4 out of 5 days. If they have a deadline, want to meet a friend for lunch, need to run some errands or whatever it's fine. Most often the conversation is about everything but business.

It makes us closer as a team and some of our best ideas come out over lunch when we're not trying to brainstorm or think about business issues.