This is a very good question. Not too many people have studied the sociology of leisure until relatively recently.
As you can imagine, theorists have a very difficult time defining what they mean by leisure. This is part of the problem.
Generally speaking, there are four basic definitions. First, some theorists define leisure as any behavior that can be restful, which can take place at work or in any other context. Second, others define leisure as any activity outside of work. Third, still others define leisure more narrowly and so exclude family obligations. Finally, the narrowest definition of leisure only considers self-fulfillment activities.
In my opinion, the second definition works best. It gives flexibility and allows one to define leisure apart from work. The sociology of work, moreover, is an entirely different subject worthy of great study.
Finally, based on any of these definitions, it would be interesting to study how leisure works in different socio-economic groups or ethnic groups. More study on this topic is necessary.