Add some walking to your day, as many days a week as possible. Incrementally (little by little) increasing the amount of walking will be best. For example, start out with parking further away from the store/office. Add an extra block as you are walking your dog at night. Take 10 minutes at work, walk for 5 minutes and then return to your office.
Your endorphins will get a healthy kick. It will positively boost your mood and energy, even if you only add a couple of steps a day.