Blogging! Blogging doesn't cost anything to do, which is important to me in a pastime. You can Blog year-round without concern for the weather or season. Unlike some hobbies and activities, blogging is safe with no risk of injury. Blogging allows you to be creative and make something that is your own. Plus I even make money doing it.
Mine is gaming, because it's all I know. Most of my life has been dedicated to video games (sadly more so than education and family values), and I don't remember a time in my childhood when I wasn't holding a console or controller in my hand. I didn't expect that in two decades I'd be an enthusiastic video game reviewer and collector, but then again I don't anticipate a lot of things.
The best thing about gaming for me is that no matter how many titles I have and no matter how bored I am with them, there will always be one that strikes out and will have me occupied for hours on end.