If you want to strengthen the body without building bulk and you want cardio to mix in with it dont bother with wasting money on kettlebells when your gym has dumbells(anything you can do with them you can do with dumbells) As for TRX AND THE LIKES THEY ARE AN EXPENSIVE WASTE OF MONEY I CAN SHOW YOU HOW TO MAKE ONE FOR ABOUT£20.
Right what you want to do is get a telegraph pole and get three other mates two on the log and two running along side. Run the log about 6 miles with the people on rest changing every 600 m.
If getting fit is what you are after then this is where it is at. This will get you fit to fight war and believe me there is no tougher test of fitness than that. It might not make you look fab in a t shirt but what is more important looking fit or being fit.
Other things you can do are firemans carry drills pick up your mate onto your shoulders and run with them.
And trust me running a 300 pound log is alot more mentally and physically challenging than swinging around on a swingy thing!
What you need to ask yourself who are the fittest people well easy soldiers, so train like they do and you wont go far wrong, oh and no they dont use swingey things or swing lumps of mettle around thay do BATTLE PT.
If you Want some real training from a real trainer then contact me on firstname.lastname@example.org
or visit http://www.dosctraining.co.uk
and i will be more than happy to introduce you to real fitness training without the gimmicks.