I personally like doing a 5-minute doodle (just random shapes), or reading a short story or article completely unrelated to my course-shakes the cobwebs off. 😄
If I were you, I’d try this combo: 2 min of deep breathing, 5 math-problems (or flashcards), then a 2-min freewriting of whatever ’s in your head. Gets me ready way faster than trying to force myself into studying cold