Whatever gets the heart rate and breathing up to a certain target level and stays there. It could be rope jumping, jumping jacks, step-ups,spinning, etc. What fits your style, something you don't mind doing (too much) for any extended period of 20 minutes or more. Might even consider the Tabata method for short burst of effort resulting in cardio fitness.
Seen that infro commerical late at night for the gazelle over and over again. Reminds me a littlle of the Nortic track. Looks a little hard on the knee joints but never having used or seen one in person, that is just my impression of the machine. Like the Bow Flex, the Gazelle can serve a purpose for a lot of people, I guess. Good Luck to your wife, baby and you.