My advice would be always have something to work on. And always do a little, even if it's just a few sentences. Free periods are a good time to get things done, especially if you start early in the free period so you're still in the flow of your last lesson. I usually do homework first, depending on its urgency. But if I didn't have any homework to do, it always was quite useful to add a sentence or two to some coursework, or my personal statement, or to CAS. That way even if I did spend the rest of the free period relaxing, I'd still have done a little work, which adds up over many free periods.