What is the least energy using thing to do to burn calories?

Well, any activity burns calories. The easiest are probably walking, riding a bike, or swimming.

If you're looking just to burn calories, go on long walks (30 minutes or more) you'd be surprised how many calories you can burn.

Since everyone burns a different amount of calories based on their weight, here's a calorie calculator:

http://www.prohealth.com/weightloss/tools/exercise/calculator1_2.cfm