if you're ovo-lacto,  you shouldn't have any problem.  every stadium i've been to always has ballpark nachos, pretzels, sunflower/ pumpkin seeds, popcorn... not exactly the best meal plan, but you won't go hungry. 
if you want something more satisfying,  you might want to try a fake chick'n patty sandwich.  you can buy the chick'n or make one of the ones off this site.  i usually put it with deli mustard, some lettuce, a squeeze of lemon, and a slice of tomato.  i don't know about it staying "nice and fresh", but it holds up fairly well, is satisfying, and easy to carry, and to be honest, i don't know that anything is "nice and fresh" in the stadium, so you can chalk that up as part of the experience.  if you have the room, grapes are a nice addition, as they give you something to munch on through 9 innings that's not too salty or sweet.
and i definitely second the water.  out here, we can't bring any beverages into the ballpark, so you might want to check ahead.  if you can't bring water, bring cash, cuz those puppies are $3.50 per tiny bottle!

Aramark is the company that provides food at the Citizen's Bank Ballpark (and many, many other places!)
Here is a break down for that park:
http://www.aramark.com/BallparkLandingTemplate.aspx?PostingID=703&ChannelID=330

CHECK OUT:   
• Vegi Cheesesteak—mushroom, sweet peppers, pizza sauce and cheese on a cheesesteak roll
• Specialty Hoagies: Vegetarian Wrap/Hoagie—roasted eggplant, broccoli rabe, roasted red peppers, sharp provolone;