Wait, no running, no ellipticals, no cardio machines? That's right! Cardio is like a box of chocolates. There are some chocolates you expect, but there are also those "different" chocolates that seem to accumulate until the very end. (Did someone say coconut?) What you find, though, is that those chocolates that aren't typical turn out to be pretty good. Likewise, cardio doesn't always have to be labeled cardio. Circuits and resistance training also get your heart rate up and work as great low-impact alternatives.
Konforti's Trainer Tip: Circuits come in all different shapes and sizes. What matters is that you choose compound exercises, or exercises that use multiple joints and activate lots of muscle. So bicep curls are out, but squats, deadlifts, rows, and push-ups are definitely in.
To get started, try grouping three to five exercises together doing 30 seconds of each with minimal rest in between. The easier it gets, the more rounds you can repeat or the longer you can do each move. Take on your first circuit using these exercises: squats, deadlifts, bent-over rows, push-ups, and lunges.