Veggies, fruits, nuts, and seeds are nutritious and easy to carry. Just watch portion size with the last two.
My co-workers constantly give me grief about the bowl of fruit on my desk while they suck down cakes, pies, candy and ice cream. I swear if you left fruits or veggies in the work kitchen they would rot. Put some heavily sugar laden desert there and its gone in 20 minutes.
Non-fat yogurt with fresh fruit is a favorite afternoon/evening snack.