Ideally, activity breaks should be as physical as possible. Students with ADHD may need a physical outlet to stay focused long-term. This outlet could be a skipping rope game or a challenge involving a bouncing ball. Anything that gets the students with ADHD up and moving around would be a positive way to spend break time.
If you don't have time to offer a physical release, try doing frequent short quizzes or games using the class interactive whiteboard.