Here are a number of seated exercises you can do at your computer:
Healthy sitting posture
Sit up on your sit bones, sacrum back in the chair, feet
flat on the floor. Refrain from relying on the chair back. Sit
Inhale, place a finger on the sternum, and lift up and
in; relax and let the shoulders fall on the exhalation.
Make a "W" shape (like a saguaro cactus) with
both forearms and upper arms, palms forward, keeping shoulder
blades drawn down the back. Keeping your chest like this, place
your hands back on your lap or keyboard.
Draw your forehead slightly back, retracted, so your chin
is in, not lifted.
Conscious breathing (Pranayama) to quiet the mind
Bring hands to prayer position (namaste) over the chest,
and lower the chin (chin lock).
Start by exhaling through the nose, fully. Without forcing,
take a deep, slow, steady and full inhalation through the nostrils,
followed by a slow steady exhalation. Your ribs will expand
to the sides and forward as you breathe in. Inhale and exhale
about the same volume of air and length of time, blowing the
air slowly out through the nostrils. Take your time, watching
Repeat twice. Finish with an exhalation, and return to
natural breathing. Release the chin lock.
Be quiet, and observe the effects.
Yoga postures (Asanas) done seated
Parvatasana: arms overhead, to stretch the spine. Sit up
and upraise your arms!
Gomukhasana (Cow face pose): one elbow up beside the head,
the other arm bent back low behind the back; try to hook the
fingers together, lower hand's palm turns away from the back.
Follow by shoulder rolls and shrugs, neck roll or a U shape
made with chin down over the chest and to each side.
Garudasana (Eagle pose): one upper arm over the other,
forearms entwined. Do both sides, stretching under the shoulderblades.
Swaying Palm: one arm overhead, palm faces inward; side
bend, stretching the waist, other arm secure on the chair for
Backbend over chair back, hands behind head; look up, keeping
a fist distance between the chin and chest.
Baradvajasana Twist for upper back, neck, and eyes: sit
up and slowly turn to each side, keeping sit bones straight
ahead, and shoulders level. Elbows bend back.
Piraformis Stretch: one leg crosses over, ankle over top
of thigh near knee, and lean over gently from the hips, keeping
back straight to stretch outer thigh and buttocks. Follow with
ankle rotations, and point and flex from the ankle. With both
feet on floor, fold from the hips to stretch the lower back.
Simhasana (Lion pose): stick out your tongue towards your
chest and roar (softly). Stretches the jaws and aids circulation
to the face and throat
Palming for the eyes: rub palms vigorously and gently cup
over eyes. Good for headaches. Repeat.
Seated silent meditation (Dhyana): listen inside to chanting
of a mantra or sacred sound; sense your breathing; or visualize
a luminous color or sacred symbol. When the mind wanders, return
gently to the object of focus.