Cardio exercise for person with bad leg?

In order of importance to good health:

Amount and quality of SLEEP.

Amount and quality of food.

Exercise in a distant third place after fixing the other two.

Sleeping lots isn't being lazy.  It's repairing your body and mind.   Try and go to bed early then get at least 9 hours each day.  It also helps you avoid bad habits with boozing and eating junk as your friends who like to do that all are doing this in the evening - the fit friends are all up early morning.  So this is the best change you can make long term.

Next important life hack is to get a daily diary of some sort and record (time and distance) of any exercise whether swimming, riding, walking, running or whatever...

Your goal should not be to hit a weight in kg.  Don't bother recording your weight - it tells nothing (much) useful.  There is no reliable way to know what is your ideal weight.  However you can always know that more distance covered / less time is a good  indicator of improving health.

Good luck.

Hand cycles are far from ideal for cardiovascular exercise - the muscle groups involved are far smaller than those in your lower body, so keeping up a sufficient level of intensity is very difficult. I second the suggestions to hit the pool. Can you swim butterfly?

If you want something meditative, although not necessarily cardio, try hot yoga. I've found that the heat really helps me to hold positions that I find painful at room temperature, due to niggling old injuries.

My advice, based on no medical qualifications whatsoever, is that you need a medical professional to get to the root cause of your knee problem. Is the joint physically worn out / damaged or is it more to do with poor muscle tone and instability. Maybe just losing weight will fix the problem. Knees are complicated joints. Doing the wrong kind of exercise could lead to further injury of the knee, hips and spine.

Also, though exercise is of course important, it is not the primary driver for weight loss. Focus on your diet, be honest with yourself, first cut out the junk calories, fags and alcohol, then look at other healthy eating changes. If leg exercises are completely out, upper body strength and core workouts are good for the ego as you will soon see a change in physique and muscle tone.

Good luck, keep us informed of progress.

One big toe up, one big toe down.
Like a drum beat, play like a clown.
A very light exercise that centre points a being. Maybe a minute drumming your hidden toes might assist your warm up, warm down.

Crawling around the house might be fun?

Doing something that is fun and utilizes movement of body with conscious thoughts of concentrate might assist your focus to achieve a freer lifestyle. Diet changes might help too.
By avoiding plastics with groceries and holding onto your plastics that your foods come in to place into the guardian of a recycling team improves your health greatly as humans ain't the smartest on this planet and we do get admired with choices of simplicity.
I live with a Queen. Rather than boot her out, a few days each year I dance with a swarm of soldiers in my kitchen. Loads of suggestions ,I've herd. They're gorgeous and I guess it's their kitchen wall.

Mimicking simple men such as the birds and the bees is often rewarding. Wealth is health. The reward is often laughter.

I like rolling around. That'll build cardio. Rolling side ways.
I don't assume well. Small exercises often. Large men are often highly toxic and targeting selfs heart might be too much might as a start.

I love swimming

When I had a real problem with my knee, I was able to use a rowing machine which I could do with just my arms. It may be that you need a specific type of machine that allows you to do this (although mine was a fairly cheap one which used air resistance).

Swimming is good, but my physio advised me not to do any breast stroke, so you need to make sure your strokes don't use your legs or get really good advice on what strokes your knees can cope with.

Finding a good physio is important as they can give your the right exercises and identify any structural issues. In my case, one set of muscles was weaker than another which had the effect of pulling my knee cap into slightly the wrong place, so I had specific exercises to improve the weak muscles.

This is probably going to be a medium term thing, so it might be many months before you see a real improvement.

Swimming, you dont have to use legs, supports extra body mass. 

How tall and how much do you weigh?

By reducing your body weight significantly by eating a calorie controlled diet with lots of fresh vegetables will improve blood pressure and probably eleviate knee problem.  

Remember your weight will have crept up over several years, so dont expect it to drop it a matter of weeks, it will take months, its all about lifestyle change.  There are many who have done it before you and you can also do it.