Suggestions: Cardio with an injured knee

My wife and I have really been ramping up our cardio workouts together this summer and have been feeling great! Lots of treadmill jogging, bike riding, and the obligatory Just Dance Wii. Then, a couple weeks ago we were simply walking through the park and she stepped on this hard green ball thing (not an acorn, but some similar seed ball). It rolled and down she went. She landed hard on the concrete, squarely on her knee. It just so happens that this knee has seen a couple similar injuries during her life. It swelled up like a balloon and hurt a lot.

To date, she's back up and roving around pretty nicely, but she is slow on stairs and will start to swell if she's up for too long. She has expressed multiple times now her desire to get some cardio in, but we've been told to keep it light on her knee until October! I'm looking for some creative ideas to get huffy puffy around the the house with minimal impact on the joint. Currently, the best things we have come up with are:

Tossing a heavy couch cushion back and forth (rough on the arms after a while)
Making waves in a jump rope to the beat of music (so light, it feels ineffective)
Playing catch with folded socks (fun, but I get more cardio than she does when it comes to chasing down those that are missed)
Just doing the "arms" portion of Just Dance (she was NOT amused ....... but I was! :rolllaugh: )

You can see the direction we are going with these, using simple objects to get in a workout. We have a treadmill and a weight machine in the other room. She does the weights but still craves some cardio. I appreciate any and all suggestions. And hey, this is a dream forum, there's no such thing as an idea that's too "out there"!