What do you generally eat at the cafeteria? Can you eat the same thing day after day, or do you need to have it mixed up?
Do you have space at your job to put non-perishables?
Do you have a fridge at work? microwave? toaster?
Soup/stew/curry - make a big batch & freeze in individual serving sizes. This way you can spread out different soups over a longer period of time if you are one of those people (like me) who gets tired of the same meal after 3 days. These freeze best if they don't have noodles or rice. Let defrost over the morning, and nuke to heat. If you have space for non-perishables then crackers, rice cakes etc can be crumbled on top.
Couscous (or pasta, quinoa, etc.) salad - if you aren't low carbing, couscous salads can go in multiple directions and can be eaten at almost any temperature. Again, while it doesn't freeze well, you can make large batches for the week and then do different dressings & toppings throughout the week to keep it interesting. Quinoa is likely most healthy of those options. (Look at other picnic ideas like bean salads.)
Salad - I used to just bring in a head of romaine and leave it in the fridge at work, carry in toppings (other veggies, protein) and have a bottle of salad dressing on my desk. Then at lunch I'd rip up a couple of leaves for salad, put on the toppings, and have a nice main course salad. I can't do this any more because the fridge at the office runs too cold, and it freezes 40% of the time. Defrosted lettuce is quite vile.
Sandwiches - I think bread always tastes better toasted, even if it was 3 hours ago. Plus then any spreads won't sink in and make it soggy or slimy. You can also bring all the pieces and assemble at the office if there is a fridge.
Veggies or carbs with dip (hummus, babaganoush, etc) - you can make this a pricey option with baby carrots and small premade dips, or you can buy the big bags of bulk carrots and cut them yourself.
And make sure you have some non-perishable backup items for when life is too busy to think - rice cakes/crackers, peanut butter (if your office doesn't have a no-nut policy), canned fish (which can go on some surprisingly good sales), etc.
And of course leftovers from dinner the night before are always a mainstay.