I work from home these days, but had plenty of years of work packed lunches and still sort of prepare myself one for the next day anyway.
I tend to do salads, some combination of - mixed leaves, peppers, red or spring onion, celery, avocado, cucumber (has to be cut chunky in my opinon!), cherry tomatoes, beans/ pulses, tinned tuna, cubes of cheese, hard boiled egg - whatever we have really. I then always have in an assortment of nuts and seeds so sprinkle a handful over for crunch. Drizzle of oil and balsamic (get a separate pot if you prefer to add separately but it never bothers me putting it on the night before). You can add extra nice bits as and when you have them - I like the cauldron falafel, various deli bits from whenever I've happened to swing by M&S, olives, roasted veg etc.
I've also seen the idea of making them in advance and keeping in mason jars in the fridge, so only one lot of prep - not tried it myself yet but supposed to keep fresh all week (I'd probably only do up to 3 at a time!) https://hurrythefoodup.com/how-to-pack-a-salad-in-a-jar/
However, current weather/ time of year maybe not conducive to salads, so agree that just making extra of your meal is the easiest way. We do that a lot too.
Or stick a couple of potatoes in the oven whilst you're cooking something else and then you can reheat in the work microwave and add cream/ grated cheese, tuna, beans - anything really.
Soup is great too - this is my current favourite - https://www.saltandlavender.com/lemon-chicken-orzo-soup/ - I usually add a tin of white beans or something instead of chicken, but I've made it the meaty way for DD and DH and it's a big hit there too.
Quesadillas are a nice change from sandwiches if you don't mind them cold - could have for dinner the night before and save some for lunchbox. You could in theory reheat in microwave but I don't think it would be great!
I used to sometimes buy a packet of bagel thins to keep at work at the start of the week, and a tub of cream cheese to keep in work fridge- then toast and make up the bagels there, adding tomatoes or other bits from home too. You could add ham, or smoked salmon if you don't think people would freak out about the smell (smell seems to be a huge issue on mumsnet!).
Not especially healthy/ balanced but sometimes I'd take one of those microwave pouches of rice - I love the uncle bens vegetable pilau one. You could get the ones that have more types of grains in if you preferred.
Pasta and pesto is one with minimal prep if you use a jar, and can eat cold or reheat the next day. Or you can be fancy as I sometimes am, and make your own - it keeps pretty well. I usually add peas, or some roasted tomatoes, green beans or broccoli also work - and extra cheese of course.
Pitta and hummus or other dips, with salad/ veg sides - is there a toaster at work you can toast the pitta?