Heating Pop-Tarts? Who does that???
I put them in the toaster, that way the butter I put on the non-frosted side can melt in better. It actually makes it a more healthy meal!
I used to take them along when I went windsurfing for refueling. That was back in the day when I had enough stamina to need food before I was too tired to hang on.
On hot days I would put them on the dash, and in the foil packets they would get as warm as if they'd just come out of the toaster.