I've had knee issues since I was 30, and was told that recumbent-bike was my best bet for exercise, as it puts NO stress on my knee.
While it's been years since I've had a gym membership, when I DID have one, I used the recumbent.
Let me tell you -- it did WONDERS for that knee!
At other times, I had one of those little portable pedal-thingies that you can pick up and move out of the way (it's just the pedal part -- you sit on your couch/at your desk, etc). Since my purpose at the time was physical therapy more than weight-loss, I made a bargain with myself: I could watch as much TV as I liked, as long as I kept pedalling.
It worked. I had greatly reduced knee pain. (BTW, if you DO opt for one of those little portable units, I recommend you do NOT buy a cheap folding one.
You want one that looks like this:
That solid body houses a flywheel, which means smoother movement, less noise, and generally better construction.
One like this:
will usually be less well-built and will not have as smooth a motion.
But if you want a full-on recumbent, I can give you an enthusiastic endorsement. They don't make my knee pain worse. In fact, using it consistently improved knee pain. And because it doesn't aggravate my knee, I'm able to get a good workout. It's maybe more dependent on your mental-state than a treadmill or (heaven help my knees) a stair-stepper, but you can definitely get your heart-rate up if you stay focused. (For me, hard-rock played loudly helps. I don't listen to loud rock most of the time, but I do for working out.)