I always cook a lot of dinner and pack the extra in their boxes and put them right into the fridge after dinner. In the morning, we take them out and send them on their way. They do not have access to fridges at school and I've been doing this for almost 8 years without much of an issue.

Here are some of my "I either didn't cook enough or they didn't really like what I made" options for when last night's dinner doesn't cut it:

bagels or sandwiches with cream cheese and cucumbers
pasta tossed with whatever frozen veggies I can find in the freezer
veggie dogs on bun
sunshine burgers
cucumber salad
cooked broccoli
baby carrots
sliced cucumbers

And some snacks:
applesauce individually packaged
pickles
granola bars
chips or pretzels or tings, whatever snacky snack we have at the moment
crackers
fruit
macaroons
string cheese
etc.

My kids both have stainless steel lunch boxes with 3 compartments, so I usually use the biggest one for lunch and the smaller two for snack or vice versa, depending on the meal.
Hopefully I've been able give at least one helpful suggestion for you. Have fun! I love surprising my kids with lunch some days...

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and I think that makes it a little easier. There are some great blogs that I look for online when my brain stops working.
I always make his lunch the night before so that I don't have to worry about it. I make a bunch of hummus every Sunday, and that's a big staple around here. I almost always send a little container of hummus with veggies, and sometimes crackers.

Right now, we are in sushi phase, which doesn't suck too bad since it's easy to make the night before. We eat rice with almost every meal, so I just pull out a cup or so and mix it with some rice wine vinegar and agave and then spread that on a sheet of nori with some smoked salmon and avocado and whatever other veggies I want to trick him into eating and roll it up and cut it.

We try to always keep hard boiled eggs in the fridge, so we can give him egg salad sandwiches, or just egg salad, on the nights that we are super tired and can't think of much to do.

If we have leftovers that need to be used, I grind them up in the food processor and then cut a pie crust into 5-6 inch diameter circles, put the filling in there and cook them for 20 minutes and he gets that in his lunch (cold) the next day.

"Fundamentally the markswoman aims at herself" DT Suzuki

Sandwiches with tons of lettuce and a small amount of cheese and sausage or hummus--I like to think of it as a salad with some bread.
Wraps with jam and cream cheese.
Pumpkin seed butter and jam sandwiches (my no nuts alternative).
Carrots and cukes or broccoli or shelled peas and lemony apples cut up and some ceasar dressing in a little container.
Apple sauce or yoghurt in a reusable container.
Left over pizza.
Any type of popular fruit that I have around--grapes, berries, melon, whatever--except for pear slices because they become slimy pear mush--ewww.
Half a homemade muffin with some butter or cream cheese on it.
Prepeeled Boiled eggs have always seemed like a neat idea but smell like hellacious farts so I don't send them because I don't want my kids to be the kids mocked for having stinky lunches...
Chunks of cheese.
Leftover blueberry pancakes.

"I'm drowning and monkeys dressed as lifeguards are throwing me anvils"
Dilbert comic strip

cold macaroni salad, tunafish, cut veggies and a wee container of dip. cold quesadilla wedges. i would check your school policy on peanut butter, before sending that, there are alot of peanut free schools now with all the allergies.
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for reals, this will make your morning so much easier.
My kid loves bringing cheese, crackers, maybe pepperoni then some olives, carrots, cukes or apples.
She also likes warm lunches, esp in winter...leftover soup, mac & cheese, etc. We got a small thermos for those lunches.
PB & J always a good standby, as are burritos (or beany-cheesy roll ups as my kids usually say)...or hummus w/crackers and veggies...