D.I.Y noodle soup jars are my absolute favourite if you have access to a kettle.
I also like to buy a packet of mince, usually turkey but anything works and season and roll into little balls and cook them in the air fryer. They keep for days and are very filling. I'll then add them to an Ainsley sachet of cous cous or salad (I prepare separate pots of salad ingredients on a weekend and line with kitchen roll to stop it going soggy), pitta, olives etc and whatever sauce I fancy. This works with falafel, chicken etc too but this is more cost effective for me.
For warm lunches I love a precooked baked potato, one of those snap pots of baked beans and a bit of grated cheese.
I also love soup in a flask or reheated, especially minestrone.
I'll sometimes make a huge batch of breakfast burritos and they'll be ready to eat by lunch time. But I do also love just a wrap with some chicken and some salad etc.
I'll sometimes just have porridge and fruit with peanut butter.
Adult lunchables of things like salami, cheese, nuts, olives , pickles, boiled egg etc and crackers.
Puff pastry pinwheels, sausage rolls etc are still lovely cold