Blended shakes, too many variations of what type of shakes you can have just make to meet your own caloric goals. I'll blend up a few shakes the night before and in the morning just throw them into a cooler bag. During the day just shake and drink.
Again depends on your caloric goals but also I like the quest protein bars, no added sugar or sugar alcohols in them, nice and small and easy to carry right in your pocket.