I haven’t seen croquet mentioned yet. Also many things could be low cardio if played that way (meaning with both people dedicated to playing it chill, possibly with some added rule around how much you can move). I could imagine, say, most raquet sports played that way, though maybe it requires enough skill that the ball isn’t going all over the place.

Here’s another crazy idea: both players wear heart rate monitors and going over a certain heart rate is a penalty.