At home I tend to have something from the following list, most of which can be packed up for a lunch:
Rye and wholegrain wheat bread, wholegrain rye bread (pumpernikel), ryvita cripsbread, whole rye bread roll, sliced harvarti cheese, camenbert, sliced smoked cheese, pepper salami, smoked blackforest ham, smoked ham, mortadella slices, ardennes pate, olives, stuffed olives, spicy pickled gerkhins, plum tomatoes, hot wax banana chillis, rollmop herring, Maggi packet soups, frankfurter sausage, tortilla wrap with tuna salad filling, tinned salmon, tinned mackerel.
If you are out, you need a good quality thermos flask or two and something for your food/sandwiches. I take two flasks, one with a soup and one with green tea - with lemon in my case. I also go to the co-op or morrissons supermarkets late in the day because they tend to have cooked chickens going cheap, that's cheap not cheep if you'll paron the pun. Obviously the meat can be cut off but leave some on and use the carcase to make chicken stock. That's tasty when things like onion or sweetcorn, maybe a few noodles are added and it makes for a good soup. Stock can be frozen too.
It's realy a matter of finding out what you like and building up a stock of things in your fridge/freezer. If you are always having to think of something the night before, it's really hard. But, it takes time and experiment to build up a portfolio of ideas. Worth doing though. You'll never walk into one of those rip off sandwich shops again. You can eat better at less than half of the price