So many things that mental illness throws at us seem out of our control. But you can control your physical fitness. Exercise can help with mood regulation and stress management, as well as provide all the physical benefits of being in shape. Many disorders are co-morbid with mental illness, and just by moving you can take steps to keep these troubles away.
No gym or fancy clothes required. Just get out and walk, or run or do some calisthenics and stretch a bit. A world of health awaits, and it feels really good.