Go for a walk or do a short exercise for at least 15 minutes a day.
Do something active that can be done within a short amount of time like rope jumping, lifting kettlebells, push-ups, or sit-ups. It might also help to split the activity into multiple shorter sessions. You can use an app that helps you with the workout, e.g., 7 minute workout, or an amazing yoga app Downdowg.
Try to create a repeatable schedule that is consistent and try to make a habit out of it. Make sure you enjoy the activity.
It can also help to create a goal or challenge for yourself, e.g. registering for a race.
Eat less refined sugar and simple carbs, and eat complex carbs instead. Try to eat more vegetables. Don't go to the kitchen to eat something every 15 minutes (it’s a trap!). Keep junk food out of your house.
Have a water bottle with you at your desk. You will be more inclined to drink if it's available at all times.
Try to get enough sleep at night and take a nap during the day if you need one.