Feel your pain! Hiking with pack, treadmill, wall squats, lunges if tolerable, elyptical, as rosinbag noted an actual stair climbing machine is awesome, though boring and a push/pull quad hamstring machine my gym has......not exciting stuff for sure but they will do it! Mt biking on grade is an excellent one as well, but 4 some reason my 2x rebuilt knee no longer likes the pedaling motion as well.
The mountain biking has helped the most with cardio, the wall squats help with strengthening the knees, and closer to season the stair master gets me ready for the mountains but I just can't do it year round anymore without excessive pain and problems.
Check out the physical therapy type of knee exercises as well, plenty of video on the internet of which ones to do.
I can't run at all - Kills the knees. Also Hiking was hurting them real bad last year, but started taking Glucosamine and Fish Oil daily - helps with the aches and pains.
I Hike with a pack, stairs at work, and Mountain Bike. I try to take a 4 mile hike with a 50# pack 4 nights a week and I do 16 flights of stairs every day (I am right at 4 minutes now) at work for an afternoon break with a group of co-workers. On the weekend I try to get in 7-10 miles on the bike. Bike and stairs are the ones that help with the cardio the most.
Its working too. Did a scouting trip yesterday to put out a couple trail cameras. 12 miles, 2600 vertical. Most of it was off trail bushwacking.
For those who get knee pain when Mountain biking. Try to keep the RPM up and use less brute power on the uphills. Really helped reduce the sore knees yeast year when I tried it - this year I dont hurt at all after riding.
Right after my knee surgery they had me doing biking, lunges, and one leg stands on a BOSU ball.
Anyways it was all low impact and got me built back up pretty quick, don't see why it would not work on a permanent basis.
I have one bad knee. Besides what's already been mentioned, a good physical therapist can show you exercises to strengthen the hip muscles that can take stress off the knee. And depending on what's wrong with your knees, unloader braces can help. My torn meniscus is on the inside (medial) part of the knee, so that's where it's arthritic and sore. I've started using a medial unloader brace, it re-distributes forces on the joint and keeps the medial part more open. You might look into that.
Also, a good therapist can show you stretches and foam roller stuff to loosen up the muscles and IT band that can put stress on your knee. That helps me.
Don't run much either, meniscus problems. I do try to build in some uphill runs every once in a while, as they are less hard on the knees. In addition to that, lots of biking, elliptical whenever I'm at a hotel with a gym, skating XC skiing in winter, and hiking with the pack (but I have to leave the heavy loads for the actual season). No more squats. Occasional lunges. It sucks.
I picked up a Schwinn elliptical, I forget the model, but it has been great for my cardio and keeping knee pain down. It has adjustable stride length and you can stair climb or run on it. Really happy with it. No impact for me it kills.