The rowing Machine is arguably the best cardio machine to workout on. The rower provides an amazing low impact workout whilst using nearly every single muscle in your body. Because of the range of movement your body goes through, the rower is excellent for joint mobility too, meaning this could also be a great choice for the more mature gym user.
Not only is rowing one of the best calorie burners due to the muscles and effort it requires, it is also a machine that is very hard to use easily and just to cruise along. When performed with correct technique it can really strengthen the postural muscles, which is a great change to how many of us spend our days slouched over a desk at work.
Rowing for a lond period of time can be monotonous so why not try doing short interval sprints for 2 minutes mixed in with some strength work and repeat these intervals with different exercises in between.