I start each gym session with a little cardio and usually I run to the gym, that way I am pumped up and ready to go when I get there. I am lucky to live near the beach and I run for at least 1k in the sand. It burns 1.6 times more calories than running on pavement and it is also a little nicer for my knees!
This is my primary circuit right now and I absolutely love it! I sandwich it in between my cardio and weights. It is hard! Your legs may be jellified after completing it for the first time but smile through it! The payoff is worth it! Start the circuit by completing 70 jump squats, then decrease to 60 butterfly situps, 50 burpees, and so on and so forth. Feel free to mix up which exercises you start with, I just like this mix! If needed rest for one minute between each move.
Now go work it out!