Well we do pay up the ass for those meal plans so I see no reason not to swipe a few things for later like sd526 recommends, and I do too. In addition to fruits, you could also make up a extra deli meat or peanut butter sandwich and have that in between your three cafeteria meals. Sometimes I'll take my thermos and fill it up with milk too. I dunno how your school is setup (I attend Arizona State) but part of our meal plans we also get food points good at the food shops on campus, which I found out sell Met-Rx protein bars so I buy them alot of the time since those points can only be used on food.
Could also goto Walmart or Costco or Sam's Club etc and get a big thing of canned tuna for dirt cheap, same with oatmeal, peanut butter, etc. I mix dry oatmeal in with alot of different things, apple sauce, yogurt, protein shakes, etc.