I just wanted to add to the stair climbing tip b/c I love doing it. I live in a flat so when the weather is really bad(like today its snowing) or I'm too lazy to go out I can just go out on my door step and get a great cardio workout!
If your a 200lb person walking up stairs for 20 mins you lose about 242 calories and 726 after an hour. Compared to normal walking at mph 3 you will only burn 76 calories in 20 mins!(remember the more you weigh the more calories you burn!)
It also becomes a full body workout. You mainly workout your glutes, thighs and calves but you still get a great ab and arm work out.
You can do this anywhere there are stairs and its cheap too!
My 30min workout: 5 mins walking up stairs and 1 min running up stairs. alternate until you reach 30 mins
Warning: Do not run down stairs you would put 7 times more impact on your body if you do.
Also, if you have bad knees this may not be a good exercise for you.