The creche at my gym would not take bubs til he was 8 weeks old ( but I didn't put him in til after his first immunisation which was at 9 weeks), so I walked for exercise.
I think walking is fantastic exercise, you can go on a few shorter trips, or a long walk, and if you push a pram it's even better for you. You can do it with friends, or with an ipod. You can go as fast or as slow as you like and you can include some hills for a challenge. The only problem is rain, but other than that, I think walking is the perfect exercise. Although, as I said in a previous post, I believe the key to weight loss is your eating plan, not exercise (although exercise helps).