What to do now?

All right, I've completed one manuscript, at long last. This particular one took a few years to complete and now that it's done, I find myself at a crossroads. Do I kick back and relax for a little bit? Or do I plunge into working on another novel? And that brings up another question altogether. There are two historical novels waiting to be written. But at the same time, I have a modern day, middle grade novel sitting patiently on the sidelines. And then there's the third book in my mystery series that's waiting its turn to exit my mind and jump down onto paper. Which genre do I choose? Historical fiction? Modern day? Or mystery?