Jabs, hooks, uppercuts... Because of the repertoire, boxing can provide you with an enormously diverse cardio workout.
You can spar with an opponent, train on a heavy bag, or shadowboxing. And don't think that with boxing you only train your arms. The footwork is at least as important and a lot of strokes come from the legs and hips.
You can also do shadowboxing with a couple of light dumbbells of 1 or 2 kg in your hands.