My personal experience:
You're allowed to do whatever you want provided you don't leave the space (except for restroom), you don't talk exam specifics (people may have accommodations for extra time or progressing at different pace), and don't use your phone.
Things I did on my breaks
-Pee. Even if I didn't feel the urge. One less thing to worry about. Being male, it's a quick process. Plus forced me to walk and get out of the stressful space.
-Eat. Easy things to eat that are "guaranteed" to not give you stomach issues. For me, that meant small green-ish bananas and clif bars. My center had issues with ants so I was glad for small, sealed things.
-Drink. Bring your own water. Again, ants were all up in the water dispenser too. I had water and a small 16oz Gatorade (old athlete habits).
-Stretch/exercise: eye exercises to relieve computer-related strain (alternate between looking at a closer object and something much farther away out the window), back and hip stretches, shoulder and neck rotations, and jumping jacks to keep blood moving.
-Breathing exercises/meditation for relaxation and focus.
Yes, I did all of those on every single break. 10 minutes is a ton of time.