I submit to you that marshmallows are incompatible with instant gratification, because they're only good when you slowly toast them over a fire until browned on all sides. Cold marshmallows (or marshmallows shoved in the fire and then blown out) are just sad and not worth the calories.
If you added that (offer them marshmallows and a way to toast them), all you've done with that test is identify the foodies :-) (I 100% agree with you about them being toasted)