Although stretching will burn only marginally more calories than sitting or standing, anaerobic exercise including calisthenics such as push-ups and air squats may continue to burn calories hours after exercise.
Here are a couple calculators and reference articles that might offer some insights:http://www.exrx.net/Calculators/Calories.htmlhttp://www.exrx.net/FatLoss/EnergyBalance.htmlviewtopic.php?f=7&t=9216http://www.exrx.net/Calculators/CalRequire.html
HIIT showing 3 times as much fat being burned despite only twice as many calories being burned:http://www.exrx.net/FatLoss/WT%26End.html
If you decide to increase the intensity or duration of your exercise sessions, do so progressively.
In the future you can get many of your questions answered by reading through relevant articles on ExRx.net. Don't be afraid to use the search feature on the site (eg: search for 'Calories Burn')