High Intense Low Impact cardio

SWIM. Takes the weight off the joints and gives you a nice workout.
If you're not feeling the desire to be in a swimsuit publicly yet, you can try cycling (spin classes are very intense and burn a lot of calories). I don't know what your knees can and cannot do, so it's tough to say. Some people with bad knees enjoy the elliptical, others do fine with weights as long as the don't lunge.
If your knees are really bad go see a physiotherapist. They would help you take away that knee pain and get to a place where you can workout again.

I've heard that boxing (as in with a video, wii, etc not with a partner) can be a GREAT cardio workout without putting any impact on the lower body.

I have a weak ankle, so I use Leslie Sansone's walk away the pounds videos. They are lower impact, as long as you can walk well, they work. No jumping involved. I would have to say they are not High Intensity, but they do raise your heartbeat enough to make you sweat if you put your all into it.

Increase your non exercise activity and lift weights.

I also have bad knees, so walking, jogging , biking. boxing is not in the cards right now.
I go to the local pool. This is low impact and even if you can not swim you can use the shallow end to do low impact walking, jogging or aerobics. Most pools even supply equipment including aqua weight and offer low impact aerobic classes.
If you are worried about being seen in a swim suit maybe go when it is an adult (18 or over) only swim. Swimmers are there just to get exercise and could care less how you look, they just want to know if you are going to tie up the lane
Do not give anyone power over you to keep from taking care of yourself!

You can also use weights and do upper body exercises in a sitting position to help strengthen your muscles.

Hope this helps and good luck in your adventure!

THERE ARE SOME AWESOME CHAIR EXERCISE ON U TUBE, I DO HIP HOPS ABS FOR AN HOUR 6 DAYS AND I TRIED THOSE CHAIR EXERCISE AND WOW WAS I SWEATING.. SO THEY ARE TOUGH BUT EASY ON THE KNEES..

If you have access to a pool, swim! I find that I get a better workout swimming than I do with anything else -- and, it's good resistance training, too, because you have to propel your body through the water. (I also jog, but I much prefer swimming -- it's just more of an effort to get to the pool...)

I do breaststroke exclusively when I swim, because it's an amazing arm workout, but I wouldn't recommend that as a start if you have bad knees. I usually have to pop my knees after a swim because of the breaststroke frog kick, but it's totally worth it.

Have to second the boxing suggestion. It's a great cardio workout. Also, maybe look into a rebounder (mini trampoline). You can get them anywhere from cheap to really expensive. I have had two knee surgeries and am not allowed to do anything high impact. I use a rebounder and do Insanity. I do the same moves as they do in the videos, just standing on the rebounder instead of on the floor. The soft bounce takes the impact out, but I still get the benefit of doing the moves. Then I'll move down to the floor for things like push ups, etc. It has really saved my workouts...and my joints.

I use an elliptical. I also just bought Cathe Fredrich's entire Low Impact series. The reviews make it sound very intense.

Spin Class! High Intensity, low impact. I burn about 400 calouries in 40minutes!