A great time to use a stretching or yoga activity would be before, during, or after a long seated activity.
Stretch Break
Slowly stretch your neck, chest, shoulders, and hips
Standing Pretzel
Cross your arms and legs, interlock your fingers, and pull your hands up to your face
http://www.coloradoedinitiative.org/wp-content/uploads/2014/08/CEI-Take-a-Break-Teacher-Toolbox.pdfHook Up
First position - Cross the feet and hands. Clasp hands and bend the elbows up so that hands are under the chin. Then breath in deep for 3 breaths.
http://www.coloradoedinitiative.org/wp-content/uploads/2014/08/CEI-Take-a-Break-Teacher-Toolbox.pdf