Personally, I'd recommend an apprenticeship rather than volunteering.
So many people I knew that went on programs like this, came out having learned nothing about the real world, treating it as a holiday with no seriousness to it that, even if they were taking part in humanitarian projects. They knew they could just go home anytime.
However everyone I knew that spent time working with a crafts person, learning a skill (even if it wasn't one they would continue with) and seeing how things like deadlines affect projects and teams, came out with a far more mature view of the world.
Personally, I didn't take a gap year. In retrospect I wish I had and had spent time in a real business, because I came out of university so naive and unprepared.