If you are ultra fat or have some knee issues the classic cardio knows as running may not be for you. In that case this post is dedicated to you.
Kettlebell shit is very easy on the knees, unless you smack the kettlebell right into your knee. The range of motion at the knee joint is very small during kettlebell movements.
Swimming is a very low impact cardio but to be honest a lot of people are too lazy to put any intensity into their swim and just float.
Walking is also not the hardest activity on the knees and it's free.
Riding a bike
Riding a bike is another good form of cardio that seems to be more knee friendly than running. Give it a shot. Make sure the bike is big enough for you and your legs extend completely otherwise you may have some joint pain.
Like it or not diet is the ultimate way to lose weight. Nothing beats diet - nothing. It works every single time. You can read my simple guideline here.
Obviously diet has zero stress on your knees. However if you have a knee injury make sure you caloric restriction is small otherwise the healing process will be much slower.