I want the weekend team retreat.
In our company we are working very busy all the year, and many people only know the people in their own team. A retreat in the lakeside resort is good chance to meet everybody and to relax together. We can do activities like canoe or games, and talk with colleagues from other departments. I think after this, the communication in the office will be more better.
Also the company pays everything, the hotel and the meals, so it is like a small vacation for free. Many of my coworkers didn't take holiday this year because the projects, so they need some rest.
The volunteer day is good idea too. But it is only one day and we work all the time, so we cannot talk much with each other. And some people are not interest in volunteering.
So I think the retreat is better for our team.