Polymetrics in laymens terms are body weight exercises but they can be taken beyond simple body weight and can include various devices. Clapping push ups and jumping squats are a good examples. You could also bunny hop with a medicine ball or pass it with a partner while doing the bunny hops. Their great for breaking up routines and also they are great to do on non lifting days. On thing though that  a lot of people get confused about is that cardio is detrimental to lifting and that is not the case. But you should never ever train high intensity cardio and weight lifting on the same day. Not cos its cannibalizes your muscles. Its just you have depleted your energy running or lifting earlier so in either workout you won't be able to lift or run as long as what you would need to in order to achieve said gains. Also it puts repairing your body into a conundrum. Before the body will repair the damaged muscle tissue it will place all its efforts into refueling the mitchancandria which act as the powersource for your muscles. So once again your muscles will gas up b4 they fix the dent in the door.That does not mean that doing low level to moderate cardio does not have its benefits on the days that you lift. Cos you only work anaerobically when lifting and you will burn very little fat however with the light to moderate cardio you will burn fat. But surprisingly heavy intensity Cairdo actually work more of your anaerobic muscles and draws power from the same stores as what lifting weights does and thats why you want to avoid it.
But like all workouts and exercises no matter how much science you put into it you still have to remember we all have different body's and needs and they respond different to every exercises. so the most important thing is to learn how to tune into your own body and listen to it objectably.

Ah, the classic "I don't want to get too big" objection. 

Crossfit.com

You'll love yourself for it.

Okay, that sounds good. I'll take a look at this and the crossfit.com site. Cheers, guys!