This phase III trial compares a multi-component mobile health and social media physical activity intervention versus wearing a physical activity tracker alone among adolescent and young adult childhood cancer survivors. Regular physical activity helps maintain healthy weight, energy levels, and health. Adolescents and young adults who complete treatment for cancer are often less active. They may gain weight and have more health problems compared to people the same age who have not had treatment for cancer. Comparing the 2 programs will help researchers learn how to increase physical activity levels over time and also how changes in physical activity levels affect health and quality of life over time.