I have the same problem, out and about all day with no access to microwave or any way of heating anything. I tried taking a rolled up omelette last week, wrapped it in tinfoil to keep a roll shape. It was good. I've made massive bowls of salad in warmer weather and flasks of various soups now the weather is getting colder. I find the flask keeps the food warm, but not properly hot if my break is delayed much. I've also taken mince and mashed potatoes in a flask, and a version of stovies with added baked beans. Chicken stew/soup with noodles also works well in the flask though the noodles do lose texture a bit after a few hours. I bought a 0.6 litre flask and wish I'd got the next size up which was a 0.8 litre, although most times if I fill it to the brim it is enough.