I feel like this is a hard one because it depends on your work setup and your personal dietary needs/ tastes. I don't have a microwave at work so I invested in a Thermos. I fill it with homemade soups, chili (this weeks rotation), and even casserole type dishes (rice, veggies, chicken and some sort of seasoning). I buy baby carrots, celery, apples, and bananas. I like salads too buy my issue is time, sometimes I don't have time to prep all the stuff needed, thus why I like fruits & veggies that don't need much work. When I have time pasta salad with a side of mozzarella and salami is great. Sandwiches can be much easier, so sometimes in summer I add those in when soup goes out. I sometimes do hard boiled eggs and egg salad. I also stock up on yogurt and granola for breakfast. I just got two bins for on top of my fridge that hold grab & go foods like little bags of chips, popcorn, protein bars, trail mixes, snack cakes, and oatmeal. If you like peanut butter a jar in your desk goes a long way (with crackers, carrots, or even a spoonful to make oatmeal more filling). I do try to keep lots of little tupper ware containers on hand as those baggies can add up and not sustainable. I used to do frozen dinners at my last job that had a microwave.
I second pinterest, even if you look at kids lunchboxes there is a lot of good ideas. I was raised on good old american food, my lunch boxes were turkey sandwiches and pb&j so I have a high tolerance for boring food everyday. I have a coworker who brings in pb&j, gold fish, and carrot sticks. It makes me laugh because the two of us have probably been eating some of the same lunch foods for the last 25 years or more. I constantly take down recipes for salads and sandwiches and keep making the same things as it really comes down to time. I would rather put my time into dinner than prepping the next days lunch.