Water.

Fruit - usually apples, oranges or cantaloupe or other melon (cut up in a container), or a banana and apricot or pear and natural yoghurt, which I then use to make my own fruit salad on the spot.
Carrot sticks.
Steamed broccoli, which I then warm up in the microwave (and sometimes eat with a bit of mayo - adds to the taste!)
Chicken salad wrap - I bake some chicken, cut it up into chunks, add some diced red or green pepper and mix it all with a bit of mayo, then it goes into a tortilla wrap with lettuce or without.
Sometimes I carry tea bags with me for quick, cheap and easy warm drink on the go (I just ask for hot water in the cafeteria).
But I must admit that I often get lazy or too busy to prepare any of that and just take apples and carrots with me, and then get sushi at the cafeteria (expensive, I know...). Sometimes I get soups - they're usually the cheapest cafeteria food and can be very tasty and very filling. The disadvantage is, of course, that you don't quite know how it was prepared and how many calories it ends up having...