Surely it should start with the question of what you have in your meal deals?
The typical ones I've seen:
Sandwiches and wraps - making your own fillers is easy, much less mayo and better quality. I love a good chicken and bacon sandwich, so will often just fry some leftover chicken bits (from a whole cooked chicken, from a precooked thigh packet, or sometimes just making one extra as I prepare dinner), a little bacon and add some pepper, leave to cool while eating dinner, then add mayo and cover to spread onto bread or tortillas the next day. You can make all sorts of sandwich fillers that way.
Salads - a little harder and more fuss to make in advance, but given that you can freeze nearly everything bar actual lettuce you can always pre-cut your veggies, freeze in portions and then just toss into some shredded lettuce the night before.
Pasta salads - perfect to use up leftover pasta and is so versatile. See above on adding veggies, just mix leftover pasta with mayo, chuck in anything you like and cover.
Sushis - perfect to take to work and so little work to make. Boil sushi rice with a bit of mirin, leave to cool, spread thinly onto seaweed sheets, add a row of whatever you fancy in it (smoked salmon, thin cucumber or carrot sticks, shredded chicken with a bit of soy and suger added, pepper, sesame seeds, tuna all work really well and take little to no time to prepare. Roll up, cut into slices with a sharp knife, done.
What snacks do you have?
What is your drink of choice?