These are the parts of the human body which commonly hold a great deal of tension. Begin by doing neck rotations and stretches to the sides, with the head resting on the shoulders. Shoulder rotations forwards and backwards can help to loosen the joints. I have found that the most helpful motion for me is to use the shaking motions practiced by advanced swimmers, who shake their arms and hands from the shoulder joint, in order to release tension. Stretching your arms upwards and outwards and rotating them can also be useful.
Many people suffer from lower back pain, which is exacerbated by sitting.
Consult a doctor about appropriate back exercises for your particular physical condition. Some of the best combine leg stretching with slow back bends. Now to the smaller muscle groups. Different pianists have different exercises which they do with the hands. I would like to warn against relying on hand exercises, because the truth is, pianists are not interested in building a stronger grip.