We usually have this problem in summer, when the sun just doesn't go down and there's no way we want to go home to cook food.

These are a few of our solutions, you'll need a supply of leak-proof plastic containers:
Yoghurt with musli, chopped nuts and sliced banana.
Mediterranean style lunch: cherry tomatoes, baby carrots, sliced ham, rolls, small bottle of olive oil etc.
Packed salad, make sure it has enough protein eg. hard-boiled eggs, chick peas, beans.
And actually - dinner leftovers aren't necessarily gross when eaten cold. When I cook I often make fast wok-type dinners: strips of chicken or beef, lots of vegetables (carrots, onions, red peppers, green apple, broccoli), spices, fried in oil and served with boiled rice. I find myself eating cold leftovers of this quite happily when I come home late a few days later