If you ask me, I guess happy people is healthy people. For instance, happy people will not have stress-related illness because they do not feel the stress in the first place. Hahaha. 

Although the study have not really found the concrete cause-and-effect relationship between happiness and health, a research by the Institute of Epidemiology and Health Care at University College London does say that happier people's walking speed declines at a slower rate than those who enjoy life less. So there is a reason to believe that there is a unexplainable link between being happy and live longer. =P

When people say they don't really know what makes them happy, I don't think they are saying the truth. Deep down they know it but they think it is too immaterial or common. For example, I am happy when I drink a cup of hot chocolate on a rainy day. While it looks so simple, when you do enjoy the thing that you do, there you go, you are happy! =)

Taking a walk in the park.

Reading a book. 

Watch movie.

Going for a jog.

Eating an ice cream. 

Helping out needy people.

All these simple activities may be enough to make you enjoy life. So you don't have to do big things in order to feel happy.