Any activity is fun ...

Only till it is not WORK :-D. Quote of the night by Sanjay Goel.

Amateurs always have more fun than professionals. Love is more fun than marriage.

To illustrate, choose any hobby u have (photography, traveling, playing a particular sport) and try to make it your source of earning. You'll be surprised to see how less u like it after a few days.

Also recommended is this related article by Scott Adams.