I used to bring in a tupperware with a piece of hot smoked salmon, some pre-cooked rice and uncooked spinach (splash the spinach with a few drops of water) and microwave the lot for 2 minutes. With a little pot of creme fraiche on the side as a dressing. I stopped doing it simply because it was quite expensive with the salmon, but definitely a good option for a few times a week. At home, DH and I quite often have smoked mackerel with a tomato and onion salad and either a bit of bread or leftover potatoes if we have them.
Soup in a thermos is always good. I'm currently experimenting with soups to make in batches for lunches. Recently did a thai style coconut and chicken soup with noodles that I'm planning to do another batch of for me and Dh to have for lunch during the week. I like it as most of my soups are veggie and I thought the bit of protein from the chicken would be good to be more filling. If you want lower cal, go for a broth version rather than coconut milk.
If you can be bothered with the faff of good salads, they are great packed lunch t options. Add new potatoes or noodles or a slice of bread to add some carbs and make it more filling. Lettuce, cucumber, noodles, spring onions, beef and peanuts with a soya sauce and lime dressing (add chopped coriander and mint and chilli) is v low cal and tasty. Our favourite filling salad (I have mine without carbs at all and Dh gets a few sliced new potatoes) is lettuce, cucumber, peppers, chicken, avocado, feta cheese and pine nuts. Dressing is olive oil, lemon and greek yoghurt mixed to taste. Add mustard and seasoning to taste.
Roasted/baked chicken pieces, if you do them (tray bake style) are very easy to toss a couple extra in and eat those the following day for lunch. Long-stem broccoli microwaves easily and well or when steaming/boiling veg add your lunch time vegetables but take them out early, dunk in cold water then add to your chicken tupperware and just microwave them all together at lunch.