This is perhaps the easiest category to get content for. If you have friends who are movie, book, or music fans, they'll probably write reviews for free just for the thrill of seeing their stuff on a site.
     Also, this is a never-ending source of content, since there are always new movies, books and music coming out. Contrast that with dog breeds-once you've written everything there is to say about every known breed, you have to wait for them to come up with a new breed!
     If you want to have some fun, you can review movies yourself. And actually, if you're running your Google AdSense site as a business, you may even be able to deduct the cost of movie tickets from your taxes. Check with a tax consultant to make sure, though.
     Music is easy, too, especially if you live in a town that has lots of live music or festivals. If not, you can buy used Cd's online, listen, and review. In fact, you could even hire a high-school kid to do this, but check their writing skills first.
     Books are a little harder, since they take more time to review and cost more than Cd's and movie tickets.
However, you can paraphrase what others have said if you're clever and don't violate copyright.