Walking is man's best medicine.
~ Hippocrates.

Walk as much and as often as possible. Double benefit: fresh air and exercise. Walk to your colleagues in the office instead of writing an e-mail. Walk and talk with friends instead of meeting for a meal.

Another good thing that has a positive impact on your health is to walk your talk.

Stairs

Walking in stairs is one of the best "free" exercises. Do it whenever you can. Thus you will not need huge amounts of boring training passes.