I do a few things to manage stress. Practicing the cello is something that I try to do very often because it keeps me calm and uses a different part of my brain. I also go to the gym often and like to take walks through nature.
I find that exercise, a reasonable (not too much in the way of fats/sugary food) diet and having a regular sleeping pattern with 7-8 hours of sleep are good for managing stress. Doing these also help me to concentrate and work more effectively, which helps to avoid being in very stressful situations in the first place.