Try Taichi or meditate.
Listen to classical music.
Or draw something.

How to breathe to relieve stress:

This activity takes literally ten seconds and you can repeat as much as needed. Breathe in slowly for 3 seconds through your nose. Don't vary your breathing intake, try to inhale equally throughout the 3seconds. Once the 3 seconds are up, hold your breath for 1 second. Then, breathe out for 5 seconds. Breathe equally throughout the 5 seconds. After that, hold breath for 1 second. Repeat over an over. Optimal is to get six full breathes in one minute.

Notes: you don't have to completely go to max lung capacity to do this, in fact it's better if you don't. The key is the control. Controlling your breathing can control your heart rate and by slowing your breathing down like this will automatically make it go down. It may take some practice if you have weak lungs, but most likely not. Try to do this before sleeping or when you wake up, right before or after exams, before a competition, etc.