Yoga Journal Office Yoga - Stress relief you can do at your desk. Day 1
This is a great way energize your body when you are feeling tired and sluggish. Arm raises allow you to open your chest allowing more oxygen into your body. You can watch the video for a demo.
There is also a tip here for creating more core stregnth simply by sitting on the floor for a few minutes a day. Maybe use this time for some deep breathing or meditation. This is a great way to center yourself when you are feeling stressed.