Depends on your food boredom threshold. My current lunches (for a few years now) are a tupperware pot of raw veg (always carrot, cauli, tomato and mushroom, plus other stuff like radish peas, edamame, baby corn depending on what’s in the fridge), plus a bag of crisp type things. Then a banana mid afternoon. But you would have to prepare that and bring it in.
when younger, and feeling poor, I would buy a small loaf of bread and a pot of coleslaw, and just make coleslaw sandwiches, replacing bread and coleslaw as required. You do have to trust that your coleslaw and bread won’t be nicked by co-workers (although bread survives quite well in an office drawer for a couple of days, I wouldn’t leave the filling out of the fridge).
If you need variety, then it might be more difficult.