When I had/have long days at school/work I bring the following (on normal length days I bring this kind of stuff too for lunch):

Yougurt, you can even bring a small ziploc bag of muesli to throw in. Makes a very filling meal. You can also throw in fresh fruit and chopped nuts. And yougurt can stay out of the fridge for a whole day without going bad. Eat it for lunch if you're worried.

Sandwiches. Only takes me a couple of minutes in the morning to make up. Slices of whole grain bread with some slices of cheese or ham or salami and slices of peppers, cucumber, tomato, lettuce etc. Also will keep all day, although by late afternoon the cheese may be a bit sweaty.

Hard boiled eggs. They keep all day without problem and are very filling and nutritious. Feels like a proper meal full of protein even when you're just grabbing it on the go. Boil up a batch of eggs on the week end. They'll keep all week.

Lots of fresh fruit. Bananas fill me up the most, but apples, pears, oranges etc works too. Fresh veggies are good snacks too, although with my Chron's I found that fresh/raw carrots were a bit too tough to digest. Fruit of course keeps for several days...

Salad. Mix together a big salad one night when you have time. Throw a portion in a tupperware box and take to work. Bring your dressing in a separate little container so the salad doesn't go soggy. Keeps all day too. And so you don't get sick of eating the same salad day after day, mix the stable stuff ahead of time and then each day throw in one thing different on that days portion. Such as a hardboiled egg, or a handful of nuts, or some chopped cheese or whatever else you fancy.

Trail mix. I make up a big ziplock bag of it on Saturday when I come home from the grocery store with lots of yummy stuff. Mix in whatever you like such as dried fruit, chopped nuts, chocolate (dark works best in my oppinion), pretzels.

Granola bars. My favorite here in the UK are GeoBars. Fair trade granola bars either with chocolate and raisin or with apricot. Very yummy and nutritious.

I also used to cook big meals when I had time and keep leftovers to be nuked later when I didn't have time. Freeze in portion sized tupperware and nuke straight from the freezer when you get home late. Meal in minutes. Soup is excellent for freezing and very easy to make. So is risotto or stew. Such tupperware boxes of meals can also be brought to work and nuked of course.