Here is a recent article on recess... and simple tips to improve it it. I love the idea that you can give kids a bit more freedom- but with clear intentionality so that you know what you are trying to achieve: healthier, happier, more focused students who learn to have sustained play and be problem solvers.
http://www.washingtonpost.com/blogs/answer-sheet/wp/2015/02/04/how-schools-ruined-recess-and-four-things-needed-to-fix-it/