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. In order to function properly at the piano, our fingers need to be agile and flexible, able to release instantly and articulate cleanly. In fact, using grip machines is akin to dancers using heavy weights. Too much muscle mass prevents dexterity, instead of enhancing it. Stretching exercises for the hand and wrist can be moderately useful, but we must not lose sight of the anatomy of our pianistic mechanism.