Hi
My work ( im on maternity leave now) involves me being either in atruck or out on site from 7 am to 4 pm , we couldnt heat up our food and only had our cooler bags to keep things cool, i found that in summer, sandwiches were ok, or cold jaffles , little sausage rolls etc as long as they didnt go to soggy in the heat of the day, although im guessing if you DH is driving , he prob has air con on on hot days . Winter nothing beats hot soup, a flask of boiling water and a cup of soup with a bread roll etc.
What the girls above have mentioned is great, although im very jealous of the little oven.

Hmmmmmmmmm wonder if i kick my passenger out i could fit one in.