I'd recommend low impact for the sake of your knees.  Stationary bike and Swimming would be best for your knee.  Also do some leg strengthening to help your knee then add walking into your routine.

We just bought the wii fit a couple days ago and it's pretty fun.  It really doesn't seem like exercise when you're competing with your kids and hubby. lol.  I gained a lot too over the years (80) and now I'm diabetic.  Check with your doctor and talk to a nutritionist to help you eat better.  After I started eating better the weight started coming off slowly.  I used to skip meals all the time (bad) and then ate a ton.  Eating healthier restarted my metabolism which was so slowed the weight wouldn't come off.  Now, I get hunger pains like I should when I'm late for a meal. I lost 30 pounds last year, just from eating better.  I added stationery bike and wii fit "this week".  To break it down: I quit smoking 2 years ago.  Then diagnosed diabetic.  SO 1. quit smoking 2. focused solely on eating better and drinking water 3. added exercising slowly.  It's taken a while but trying to do it all set me up for failure and I gained weight.  Spread it out and do your best, then it's not so overwhelming. Don't feel guilty.  Just taking the stairs is a pat on the back, not skipping meals, walking around shopping, etc.  Good luck to you.