I love blogging! (...but I do have my "dry" periods when I'm too lazy to type).
I think of a blog as more of a virtual journal. You can be a chronicler, meaning you can write about what happened during the day from your own point-of-view. You can write about a specific event you want to remember. You can write about how you felt about what someone did or something you saw. You can write about strange thoughts that you want to flesh out. You can vent. You can do memes and surveys. You can post links to poems/images/songs/videos/etc you like/dislike and your reaction to it. You can write about your feelings towards a certain subject. Basically you can write anything you want in your blog!
However, you have to keep in mind that blogs are "public" and most of the time (depends on your privacy settings), can be viewed by ANYONE on the internet. So sometimes you might want to be careful when blogging maliciously about someone.
If you're ever stumped on what to blog, there's plenty of "pull-out-of-a-hat" tools/subject sites on the internet. I'm usually on xanga, so I use this site:
http://www.xanga.com/featuredquestionsI hope that helps.