Remember in gym class as a kid where they would make you do jumping jacks as fast as you can for 30 seconds and you felt like you could go forever? Well you probably wont feel as fresh at the end of 30 seconds this time but doing jumping jacks is still a great exercise to warm you up and get your heart rate up. They are also a great whole body exercise and can strengthen across the board.
Start in standing and at the same time, bring your legs out to the side and arms up above your head. Bring yourself back to your starting position and then repeat as many times as you can in 30 seconds.