It is simple, easy and fun to incorporate workouts in an office set up. Many good workouts require not much space and no equipment. Sitting stationary at a point is not good for muscles. The sedentary jobs also lead to obesity. So best is to move the muscles in whatever space we have.
Get up from your chair and work on your laptop standing and in squating position. This works on thighs. You will get buns of steel.
Replace the office chair with a gym ball/ exercise ball. This is great for core and posture. Since you are not reclining or leaning back, your core is automatically activated and engaged while trying to maintain the balance.
You can do push ups, tricep dips, squats, lunges, even ab crunches all using your chair or table.
For back- it gets stiff while sitting in one position. It is important to stretch and strengthen it. While sitting in your chair, turn around.(as shown in pic) This twist come from the base of your spine. It is an excellent stretch for your back. Similarly bend sideways.
Also work on back extensions, either lying on mat or on a gym ball. This strengthens the back muscles (erector spinae).
For stiff shoulders, just roll your shoulders time to time, sitting on your chair. do forward and back rotations. Also you can alternately lift arms overhead.
Standing in tadasana is a good posture to help to release stress. Also while seated in your chair you can do some breathing and meditating exercises like kapalbati or anulom vilom.