In the US, you have those disposable, reusable containers. They are very handy for salads.
If a vegetable salad is not enough for someone for lunch, try making a pasta salad. The amounts of each ingredient can vary according to taste. You can make a big bowl on Sunday (of for Shabbat lunch, if you wish, and take some each day to work.
Pasta
sundried tomatoes
fresh tomatoes
artichoke hearts or hearts-of-palm
pitted olives
tuna
feta cheese (if you want something chalavi)
whole chickpeas or other beans
basil (fresh or dried)
fresh chopped parsley
salt pepper
a bit of olive oil. If possible, use the darkest green extra-virgin olive oil.
Mix it up and you are ready to go.
Vary to taste–feel free to add anything in the fridge that sounds good with it.