I've taught for 18 years, and have kept a book blog for over two. I realized that a wonderful real world application for opinion writing would be a book review. It combines the opinion writing that is emphasized in the new Common Core Writing Standards combined with the new CC reading standards.
Students can be taught the different parts of the book report project as mini lessons in Writer's Workshop. I teach this over a three week period, with the book report as the final project. Each section such as the hook, summary, and opinion paragraph were taught as a focus for each week of Writing Workshop.
This was just completed using the Common Core standards in Sept. 2013. This project can be used all year.