Last year I began "student choice" month in February. (From August to December students are preparing performance pieces. In January, we're finishing up old books and supplemental material.) February is the perfect time to begin something new.
For four weeks my students choose their own material to work on in lessons. They can choose anything that isn't too far beyond their level of ability. Anything. If they don't have something particular that they want to learn, we'll just continue with our regular lesson material. Needless to say, I sure teach my fair share of Fur Elise and the Canon in D! But this year, my 5 of my students actually requested such pieces as Beethoven's Pathetique Sonata, Rachmaninoff G minor Prelude, Elton John's "Don't Let the Sun Go Down on Me", Tom Jones "Its not unusual", and Liszt's Hungarian Rhapsody.
In the end, my students are very connected to their music; they read better, count better, and really have a great time learning. It doesn't get any better than that.