Ask HN: What's the right balance of "goof-off" time?

I'm a strong believer that having some fun activities integrated into a work environment are very healthy. Obviously there's a fine line between a quick, energizing goof-off (eg: Foosball match for 5-10 minutes, lunchtime Caracasonne or Dominion games, etc) versus utter distraction (lunchtime movies or Catan games). Curious to hear any best practices from others...