You can get an excellent cardio workout with a Heavy Bag and Jump Rope.
Try punching ballistically for 30 seconds (at least 3 punches in every combination!) followed by 30 seconds sprint on the rope. Do this for a 3 minute round. Do 4 rounds with 1 minutes rest and build up to 6 rounds.
When you are comfortable with 6 rounds, drop the rest to 30 seconds between rounds. Then you can start adding minutes :).Hope that helps.
I also tend to find the usual cardio fare boring but I really enjoy the heavy bag for its cardio benefits as well as the obvious stress releasing benefits of pounding on the heavy bag.
The workout you describe above is pretty much identical to what I do with some minor variations. I have a wristwatch with a stop watch function, which I set for 3 minutes (the length of a boxing round)then I hang it up on a hook in my basement/ workout room near my heavy bag.
After a general body warmup of about five minutes, I’m ready to start. I start the stop watch, and then proceed to hit the heavy bag anywhere from 30 to 50 times, using various combinations, then I jog on the spot for about 30 to 50 times (I used to hit the bag, and jog for a certain amount of time each, but found it easier to just hit and jog for a certain amount of reps…in this way I’m not constantly distracted by looking over at the stop watch to see how much time has elapsed).
So I alternate between hitting the bag and jogging on the spot until 3 minutes has elapsed, and the buzzer on my watch sounds. Then I quickly restart the stopwatch (which can be done in less than 2 seconds), and then I do either a set of crunches or leg lifts for 20 reps, then jog on the spot until a minute has elapsed…which counts as my “rest” between rounds.
I do 5 rounds of 3 minutes each for the heavy bag/jog on the spot combo’s…along with the 1 minute “rest” periods between rounds.
I really like it as I don’t find it boring…and it is an extremely hard (for me anyway), but effective cardio workout. I used to do my heavy bag workouts for 15 to 20 minutes straight i.e. hit the bag constantly, without any jogging on the spot, or “rest” periods, but I found that by the end of the workout, my shoulders were so wasted that I could barely lift my arms, and I had so little strength that my punches were more like love taps. I find my current manner is more effective, in that my heart rate stays elevated for the entire process, but I’m not burning my shoulders out, and my punches have power from start to finish, without taking too much out of my shoulders.