run,run,run! also do 1 push-up then roll over and do 1 curl up, keep adding one each time u roll back over for push-ups until you finish doing ten in a row of each.
Forget doing situps or crunches. These are a complete waste of time. You could do 500 a day for a year and all that would happen is that you'll get good doing them. Do exercises that involve using your hold body and avoid exercises that have you sitting down or lying down. Your body does not work this way! So do stuff like wood chops with a medicine ball or cables, plyo jumps, squats, chin ups, hand stands. Think sprinter not long distance runner (unless you are training for a marathon). Doing interval sprints (you can even use a skipping rope) will get you very lean and boost you metabolic rate. Don't fall into the aerobic fat burning nonsense. If you want results, do intervals and circuits. e.g. pick five or six full body exercises and perform each one for 40 seconds followed by 20 seconds rest and move onto the next exercise. Don't fall into the trap of thinking that running for longer will get the best results. If you are running, try and run the same distance in less time.