My son eats very little food from a jar. Here are two methods that I use to take homemade food along on day trips:
1. When he will be eating the food within a couple of hours, I heat the food very hot and then put it in a small Foogoo Thermos (They are great! You can get them at Wal Mart) It is usually still pretty hot a couple of hours later.
2. When I need the food several hours later, I pack the cold meal in a small cooler bag with an ice pack. I also pack a thermos of boiling water and a plastic container to heat it in. When he is ready to eat, I take out the meal (usually stored in a glass canning jar) put the jar in the container and pour the hot water in. Voila! Instant water bath. Stir it occasionally. Depending on the amount of food, it is warm in about 10 minutes.
If I know I will be in a restaurant or a starbucks or something, I don't bother bringing hot water, because most places are really nice a about providing hot water.
If I am in a rush and don't have time to pack everything, I grab an avocado and a banana a small plate and a fork. When mealtime comes, I mash together 1/2 avocado and 1/2 banana. This is a big favorite and much more sustaining that pure fruit
Hope these ideas help!