For me, it depends on the situation. I covered an AP World History class for a week in June. Their teacher was out grading the AP exam, and the class' assignment was to watch movies (they were done for the year). I so needed to do something to keep me sane, and knitting saved me.
But there is this 7th grade science class where the teacher usually leaves a video for the sub. (I've subbed for her a couple times.) The class has a worksheet to answer as they watch, and they need someone to keep after them, reminding them when answers to the questions are coming up. Those days I'm way too busy to have time for knitting.
The week before spring break (the week I blogged about last week) was one such week where I needed to do something with my hands. By Tuesday I had exhausted my usual keep-my-mind-busy-while-the-class-does-what-it's-supposed-to-do activities. (Some weeks I don't even touch those.) I needed an activity, and fast.
So, I spent the week (Wednesday-Friday) knitting earrings.