Going to the gym doesn't have to mean you need to master the newest machines or lift free weights. In fact, sometimes the only thing you need is you. If your primary goal is to lose weight or get strong, keep it simple by utilizing your own body weight to work out.
There are hundreds of ways to challenge all systems of the body through body weight exercise. For example, the circulatory system - aka the cardiovascular system - is stimulated through longer duration, moderate intensity exercises such as running, jumping jacks, power skipping or even just jumping up and down or side to side.
The muscular system, meanwhile, can be targeted through strength exercises such as plank chest taps, push ups, lunges and squats. Each of these targets specific muscles to improve strength.
Another benefit of body weight exercises is their inclusion of balancing components. When using machines, the body doesn't need to balance any weight because it can only push or pull the weight via the cable. Because the cable guides the movement, most of the balancing action is eliminated.
For example, let's look at the differences between doing a leg press on a machine versus doing a standing squat. The machine is set up for a "generic human," and a cable guides the resistance, thus eliminating any balancing possibility. With a squat, however, you're in control of how the weight is distributed by having proper form from your feet all the way up to the crown of your head. You're forced to balance your own body weight. The same principal holds true when comparing machines to free weights, as free weights still challenge the body to balance.
There are many ways to progress any body weight movement, such as increasing the number of sets and/or reps. You can also shorten your break time, lengthen the duration of the exercise or add supplemental weight when appropriate.
A great way to include balance in your body weight routine is with a TRX class. These classes use body weight to provide resistance and help promote good balance.