I only work 3 days a week but take my own lunch about 99% of the time. Some great ideas on here. I'm a community carer so eat in my car, no access to kettle, microwave, etc.
In the winter, I bulk cook various soups (favourites plus current offering are spicy butternut squash, leek and potato, cream of tomato, cream of chicken, carrot + coriander, curried butternut squash)
I sometimes also take lentil dhal or Chana dhal, black bean chilli or beef and vegetable stew.
In the summer, I have some form of salad (ham, egg, tuna, leftover chicken, cheese). The more colourful the salad, the better for me. Other options I like are tuna mayonnaise pasta salad, orzo salad, bulgar wheat with feta and roasted veg, tabboulah, falafel with crudités + hummus + pitta, mackerel pate with oatcakes or crackers.
I'm not the biggest lover of sandwiches but when I do have them, it's usually simple involving cheese (with cucumber, tomato, pickle or salad), sometimes tuna mayo and cucumber or onion. I also love wraps in the warmer months with chicken salad or, falafel/hummus/salad. Take a side salad in a separate container. Sometimes olives or steamed broccoli/green beans/sugar snaps.
I don't really enjoy leftovers for lunch (happy to freeze them for another meal) but prefer soup/salad/occasional sandwich or wrap. I also recommend the wide neck Thermos to keep food hot, or I take a freezer pack in the summer to keep food Cold.
I always take 2 pieces of fruit, whatever is in season, and a packet of crisps, though I don't always eat the latter. Sometimes home cake if I've made some.
Things I'm going to try based on this thread; samosas and DIY pot noodles!