gets the heart pumping... high intensity cardio... i'll box a few nights a week, no sparring, just heavy/speed bag, some leg work... its important to keep your legs moving when hittin the bag to get a good cardio workout out of it...do NOT stand with your feet planted and just wail away at the heavy bag, keep your puches controlled and in combos, breathe right, dance round the bag and you'll get a good session... dont go nuts with haymakers for 1 minute, cus you'll tire yourself out, you need to build stamina and go for like 4 3 minute sessions with less than a minute break btw them...
try jumping rope, my old trainer in college at the Y told me 5 minutes of constant jump rope = 15-25 minutes on the treadmill... i'm not sure if thats true, but i do about 3- 5 minutes sessions on boxing day and it definately feels like ive been on the tmill for an hour!