I do not really know how much stress you should be putting your knees through because I am not a medical professional, so here are two videos that don't require you to put any pressure on your knees. There are also a lot of different core videos that do not require much pressure on the knees that you might find that you like. Sarah Kusch is a great expert who offers a bunch of nice core work outs. These seated yoga vidoes are a good way to get a nice little stretch in and there are a couple of these if you search for "seated" or "chair" yoga. The last clip is to Angelo Dela Cruz's program called "5 To Thrive" that has some really great mobility excercises that are low impact and help you get moving each day. If you are looking for something a little more intense, Ben Zorn has a really good Low Impact program called "21-Day LIT Challenge". This challenge could potentially put some unwanted pressure on your knees so beware, but it is a good program to work towards as you get stronger!