kiwi Do you have access to a microwave at work? We eat a lot of rice, so I often put some in a box. We keep some chopped frozen veg, eg peppers, carrots, peas, sweetcorn, throw some handfuls of these in and they defrost by lunchtime. Cheap frozen prawns can be chucked in too and also defrost, alternatively any leftover meat you have, or just have veggies, and throw in some sauce/condiments if you want (leftover gravy, pickle, relish, chutney, mayo...). It's really quick but gives you a proper meal at lunch that is tasty, filling and healthy.
When I lived alone and so didn't have much in the way of leftovers I would make tuna pasta - boil pasta, mix tinned tomatoes with basic herbs, tuna, and some cheese if I was feeling adventurous 
, mix it all together and take in for lunches. It lasts a few days in the fridge at least, or can be frozen. Or tuna pasta salad with pasta, tuna, mayo, sweetcorn and spring onion.
Tortilla wraps are also really easy, quick and interesting - throw in some frozen veg again (I keep saying frozen as it's already chopped and quite cheap to buy so minimal prep), plus leftovers, or cooked meats, sauces etc. You can make up several, wrap in foil and freeze. Then on a day where you feel like having one it will usually defrost by lunch, or pull it out of the freezer the night before.
Ploughman's too, easy to prepare but makes a nice change especially in warmer weather. You can buy those homebake rolls quite cheaply, throw one in the oven either in the morning first thing whilst you dress or the night before - or just buy some crusty bread rolls if they're within budget. Block of cheese, sliced ham, grapes - easy things to have in the house that will get used, then just cut off a chunk of cheese, take some slices and a handful of grapes and throw in a box (everything gets thrown into the box here ). If you have little pots, like weaning ones, they're good for some butter and chutney. Have a plastic knife in the box if there's none at work.
Soup. Everyone likes soup, no? Condensed is very easy if you don't have anything prepared.
One MNer gave a tip for breakfast on the go which I thought was a really good idea: buy croissants (cheap or reduced will do), slice down the middle on the side and stuff with ham and cheese (or anything else you might fancy). Wrap in foil and freeze. When you get up turn on the oven and throw one in, within about 20-30mins whilst you get ready it should also be done and then you can just take it with you (NB: remove from the oven with a glove or somesuch however!). I take granola bars or flapjacks as I can't eat breakfast at home, which I will be making from now on.
Don't forget to have a reusable bottle to put a drink in (juice, water, Coke, whatever you like!) as it will be much cheaper than buying a drink out. If you like sweet treats then buying multipacks and taking one with you will also be much cheaper than buying out. Bananas, apples, pears and satsumas/tangerines etc are easy to throw in the bag.
Generally speaking, we just always cook a bit more than we need for the meal (force of habit, we don't seem to know how to cook just for two), so I just put leftovers in a box and reheat at work. It's great! But I have to have a 'proper' meal at lunch, I can't get by on a regular sandwich 
Sorry it's so long 