Exactly walks are good, I love going for a morning walk, makes me start my whole day feeling fresher. I'm fully awake when I get to work and since my work is constantly mind engaging it needs to be fog free.
Walking is not only cardio (you do have to keep the pace peppy to get the heart pumpin) but it is also the least expensive form of exercise, requiring no gym memberships or special equipment beyond comfy shoes.Treadmills for me = BOREDOM. Any sidewalk or nearby trail is vastly superior IMHO.