For keeping food warm, check out the nearest Truck Stop. They sell all sorts of appliances that use an inverter that plugs into your cigarette lighter to keep food warm (and even cook them). From crock pots to little warming trays to even coffee makers. Truckers use them all the time. I received this tip when we were looking for some appliances that we could use while car camping. We ended up actually just buying an inverter and bringing our own small appliances along, but they have some pretty cool little food warmers and such at the truck stops' stores. (I do hope you're asking for foods to just eat in the car and not while actually driving. )
Something we give dd to eat for snack at school (where she eats it on the playground, so, on-the-go) are the squeezable yogurts and squeezable whole fruits (mushed up). Those might work for you. In addition to whole fruits and veggies that travel well, there are a lot of pickled foods that would be easy to tote along. Dd eats olives and pickles as a snack when on the go. I also like the wrap idea above. You can also do different salads in a wrap, so you could not only have a sandwich wrap, but a side dish wrap. Smear a little dressing on the wrap, lay down leaves, then put on shredded carrots, cheese, other vegetable strips (peppers, zucchini, etc.), then roll up and you have a portable salad. We really like lettuce wraps on hiking trips as they are a little different from your usual veggie sticks. Really, you can fill a wrap with just about anything, not just meats, cheeses and veggies.
Another thing we like to bring along when we're camping or hiking (sorry, we don't eat meals in the car, so I'm thinking of other places we eat on the go that might help) are bite-sized hors d'oeuvres that travel well. For example, you can bake mini-quiches in a mini muffin tin and the sky's the limit on what you can fill it with. Just eggs, some cream, and whatever other ingredients you want to put in it (ham and swiss, turkey and cheddar, various veggies and various meats). Those are great at room temperature, too. You can also do baked wraps with puff pastry. Roll it out, smear it with whatever filling you want, roll it up, then cut into spirals and bake. I imagine something like proscuitto, asparagus, and provolone pinwheels would transport well and it's pretty balanced. Again, you can put whatever filling you want in it. Hope some of those ideas help.