get some boxing gloves 10 - 20 ounces and then do some boxing, then you will have a real work out :), the gloves will knock you out in less than a minute.
Well, nothing is a good substitute for the real thing :). Training for MMA is also a good workout too. Especially those frickin stairs runs.... :(
But for the average overweight (or underweight) gamer, running the training exercises in Wii sports can definately help them get some exercise, without them even having to leave their living room.