If you know a day is going to be one of those 'on the go' days in advance it can help.
Chopped raw carrot, pepper, cucumber, etc are a good base for a lunch
Relatively low carb crispbread is an easy to eat option, look for those which have the highest fibre content (often the own brands are best). A little cottage cheese with some chopped radish is an option to hold the crispbread together. You can use either low fat or full fat cottage cheese depending on how calorific the rest of the meal is.
Avocado and nuts will provide more sustenance (ie calories) and with much more nutrients than the usual 'on the go' offerings.
All you need is some tupperware, a bit of time the night before and to keep stocked with these kind of foods.