I invented an interesting strength exercise that you can do at home without any weights. Now keep in mind that I am not a doctor or an expert in sports medicine or any kind of expert at all, so I cannot guarantee that this is safe. But it seems safe, and it’s quite comfortable as long as your knees are good.
Make sure the room is well-ventilated and free of cats. Take a long, light pole of any kind and tie a string to the middle of it. Tie the other end of the string to a set of keys or some other jingly object. Adjust the string’s length so that when you hold the bar up horizontally in front of your face, the keys are hanging an inch above the floor.
Now slowly squat down as far as you can comfortably, and come up again to the starting position, without letting the keys touch the floor. The idea is that you’re lowering and raising your body while keeping the bar at the same height. Count how many times you can do this, and the next day, try for at least one more.
Edited to add: I think it helps me keep my back straight if the pole is as light as possible.