Catrin,
I've been thinking about your question all day and honestly can't come up with a good answer--at least not without the input of your PT as to what activities might cause less, rather than more, irriation to your hamstring and quad. Obviously, cardio assumes some lower body movement, because it's otherwise pretty darn hard to get your heart rate high enough. Hopefully, the PT will have some good advice.
In my opinion, this is an excellent opportunity to look at the bigger picture of your health and well being--if you haven't already. Clearly, exercise is important to your health and all other things equal, it will hopefully remain your go-to stress reliever. But what you're experiencing in terms of an injury is not all that unusual. I would imagine most of us, for one reason or another, have experienced interruptions in our exercise routines.
I certainly experienced that myself over the past year. I won't lie; it has been incredibly tough. I'm very routine and habit driven, so to have my whole world turned a bit upside down has been very difficult to deal with. I finally had to accept that I need to have a few more things in my arsenal other than riding and exercise to help battle stress, anxiety and depression. For me, it ended up being cooking, gardening and volunteering. I also found that yoga is a lot more adaptive than cycling to my aches, pains. While I miss riding 200 miles a week, there is some comfort to be found in being resourceful and adaptive.
Your current injury likely won't be your last interruption unfortunately. So, while I hope you can continue to exercise--with your PT's blessing--through it, I would strongly suggest that you develop other avenues of stress relief. Meditation, Tai Chi, restorative yoga, knitting, drawing, photography, journaling, reading, etc., etc. Whatever gives you a sense of enjoyment and calm.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher