My ideas might be pretty standard/obvious, but due to my work/study I have many days that I have to bring enough food for a whole day. Some of the things I eat most often:
- pre-weighted portions of nuts, and/or seeds, and/or dried fruits,
- whole grain crackers with some peanut- or nut butter, that I put in a small plastic container so it's just enough for 1 day so I can't over-eat

and because the original big glass jar is too heavy to carry around,
- raw vegetables like carrots, cucumber, bell peppers and cherry tomatoes, also great with some nut butter or bean dip,
- fruits,
- whole grain wraps, with a small plastic container of hummus or bean dip, and some extra beans or legumes, which I 'assemble' at lunch time,
- shaker with a scoop of protein powder, so I only have to add water (or other liquid if I have it with me),
- I also often make big batches in advance of home made protein bars and home made 'lara bars', which I keep in the fridge or freezer and are ready to grab when I need them.