Hi,
I also have bad knees. I started out my exercise routine riding an exercise bike, the kind with the handles that also move. I rode it for 10 minutes at first, then worked my way up to 20 minutes. Now, I am able to walk. I started out just going around the block. Now, I can walk a mile or a little more at a time. Lots of days, I walk a mile twice in a day. I have also recently gotten a mini-trampoline. I am starting out slow on it, but it is supposed to be even lower impact than walking. It is sure a work out, but lots of fun, too. I just put on some music and kind of jog in place on it. I don't do it very long at a time right now, but hope to work my way up on it, too. It doesn't hurt my knees. I think that as you build your leg muscles up it takes some of the stress off your knees. I also bought some two pound hand weights, and I use them sometimes to exercise my arms. I also do stretches and crunches, etc. sometimes. That doesn't bother my knees at all. I also have other joint problems, but the exercise has really helped that more than it has hurt. Some days it is really hard to walk the mile because of pain in my left leg, but I keep going and I make it. Some days it is not as bad. Every day, it makes me feel better when I exercise both physically and mentally. I have more energy than I have had in a long, long time.

I hope you can find what is right for you. Just try some things a little at a time and work your way up slowly whatever you decide to do. Just get moving some way or another. It will make you feel so much better.
Have a great day!
Martha