From this day forward, you may only read ONE type of book–one genre–period, but you get to choose what it is. Classics, Science-Fiction, Mystery, Romance, Cookbooks, History, Business … you can choose, but you only get ONE.
I don't know if this is cheating, but I would go for contemporary fiction - not historical novels, not romance, not mystery, not sci-fi - just plain vanilla new fiction in English. The advantage of that is a variety of subject matter and prose styles that would change over time, as styles and themes reflect the time and place of which they are a part and the writing going on around it. Just looking at a few examples of the contemporary fiction that I have read so far this year - The Magicians, Authenticity, In the Beginning, and The Unnamed - there is such richness of subject matter, setting, and prose style. In fact, I might even be willing to restrict myself to reading fiction that came out in the year in which I was reading it, allowing that I could carry over the book I'm in the middle of reading on December 31 but that I hadn't finished yet. But I'd rather not, reading for me is an exercise in indulging my intellectual and experiential fancy, not restricting myself.