Blow/clear the cobwebs away
Definition 1 of 2
Pro Tip 1/3Activity Related
Use it when an activity, like exercise or a hobby, helps you feel better. 
A game of tennis is great to blow the cobwebs away.
Pro Tip 2/3Mental Clarity
It can express gaining mental clarity, not just alleviating stress. 
I'll go for a run to clear the cobwebs away before the exam.
Pro Tip 3/3Not Literal
Remember, it's a figurative phrase; no actual cobwebs are involved. 
Let's take a break and blow the cobwebs away.