Surely it depends on what is in the rest of your lunch box ? If that contains say a simple cheese or ham sandwich, then how about a bowl of mixed veg salad - finely chopped celery, peppers, cucumber, tomato with maybe some cooked cold rice or a simple cold pasta and tomato salad (I boil some pasta tubes or frills, rinse in cold water and drain and then simply plonk on a jar of pasta sauce and mix well). None of those foods are noisy to eat so they shouldn't offend anyone. If you are not into cooking, most of the supermarkets sell ready cooked quiches, so a slice of quiche and a little bag of salad would be quite healthy food too!
Don't forget to include a piece of fruit or a fruit yoghurt for a pud..... you can get get both low cal and low fat yoghurts that taste really nice and are not too expensive.