I've had the pleasure to train together with a former boxer, Vesna Fisher Kocoska. She is hard core. And that's how I like my work outs.
Skipping is a great way to do your cardio or warm up and you can do it pretty much anywhere. Vesna makes skipping doubles look easy, but I can guarantee it's hard work. When I skip I do about five sets of doubles or fast jumping depending how heavy my legs feel.