We are veggie/pescatarian.
Spanish omelette was always a favourite for packed lunches. Very nutritious, especially if you add stuff such as peas, spinach, peppers, chopped olives, etc, etc. plus is can be made in advance in a big batch, cut up & frozen or kept in the fridge it will keep for a few days.
Maki rolls are also good & nutritious & can be made in batches to keep a couple of days in the fridge too. I used to make 3 days worth & pack them ready with soy or sweet chilli dips etc, so you could just grab one for the day. They are very easy to make if you buy a rolling mat, which are very cheap in ethnic supermarkets, as is the the sushi rice & seaweed
I also made pasties with tuna, mashed with potato, carrots & peas, which were also popular & patties, with turmeric in the pastry & mild curry mixed flavoured veg mashed with sweet potato again they can be frozen