Kettlebell swings are good for getting your heart rate up quickly, however, you really need to study the form and perform them correctly or you could hurt your back. If you are interested, look for certified Russian Kettlebell trainers in your area.
Jumping rope is excellent cardio - and as long as you warm up with some jumping jacks, they are very safe.
Another excellent cardio exercise that is very efficient and works the entire body are burpees: http://www.youtube.com/watch?v=c_Dq_NCzj8M
I really like burpees - they are safe and really effective for cardio, and build strength in your thighs and upper body (if you do them with the pushups).
As always, if you are not currently exercising you should take it easy at first - maybe even get a checkup to make sure what you plan to do is safe for you.