Deep breathing exercises have been shown to help reduce stress and help the body relax. While deep breathing, our bodies elicit a relaxation response which includes lowering heart rate, muscles relaxing, slower breathing, and blood pressure decreasing (Source). Doing this can help students to regain focus and find more peace to get through the day.
One way to make deep breathing more visual, colorful, and fun for students is to use this stretchy toy. You can introduce this activity by modeling how to do it for the students. Eventually, you can make it apart of your daily routine and have students volunteer to lead the class in the exercise. Many students will be eager to be in charge of the class activity for a bit!
I would personally do the deep breaths in and out about ten times as it's just long enough to really feel the exercise working without losing their attention. These kind of expandable toy balls can be found at many stores like Walmart, Target, or Hobby Lobby. They are relatively affordable and can even be found on amazon. I have linked one on amazon for you here in case you cannot find one in your stores.