The most simple and one of the most efficient forms of cardio is running. Preferably outdoors, in nature. Not on a treadmill. Because it does matter whether you move relative to your ground or the ground relative to you. Three times a week half an hour to an hour of running does wonders for your condition.
If you automatically think of long endurance training when you run, you are wrong. Speed running, interval training, and Fartlek provide the necessary variation and keep your workout fun. Fartlek, or boating game, is a playful form of training in which you constantly change speed and let the intensity of your training be determined by the terrain.
And what about 'just' walking? That's also an excellent form of cardio, but it doesn't burn calories very quickly. However, walking is 'safe' for the preservation of muscle mass, which is why this form of cardio is popular with avid bodybuilders.