Exercise! - Cardio Exercises?

I want to know what kind of cardio exercises are good if you only have 30 minutes a day. Me and the girl I work with only have our lunch break to exercise and we walk a mile Mon-Fri, but we want to start walking 3 days out of the week and doing cardio the other two. Any ideas would be greatly appreciated.

Thanks Everybody!!

Walking IS cardio! Any exercise that raises and sustains your heart rate is cardiovascular exercise. As for cardio alternatives other than walking, the question is: how sweaty do you mind getting since it is your lunch hour? :)

Well we couldn't get too sweaty, but a little sweat isn't going to hurt anything. lol