Equipment-free cardio workouts that can be done at home while watching TV?

I want to start doing more cardio in my house. I'd like something I can do while watching a movie or TV but the only things I can think of involve investing more money than I can afford on a piece of equipment such as a treadmill.

Due to the low ceiling I have, jumping rope indoors and doing jumping jacks or anything else that involves jumping up and down is pretty much out of the question, so any suggestions?

When the weather warms up I'll be able to do stuff outdoors so this won't be an issue. But I want something I can do indoors where it's warm to tide me over for winter.
Equipment-free cardio workouts that can be done at home while watching TV?
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