Like others have said, go for a walk. If the weather is currently far from ideal due to the current season, I would recommend 5mins on the bike in the morning and 5 mins on the elipitical machine in the evening daily for a week.

If it feels quite easy you can increase it to ten for the next week. Then the week after, increase to fifteen or twenty, depending on how you find your progress. As the weather improves you can start going for some walks too.