officially it's not allowed, but we carried some granola bars, small water bottles, and skittles in our pockets into our last game and it didn't set off the metal detectors.
Food and sealed water bottles are allowed at Nats Park. I often bring a small backpack with water bottles and peanuts and room to pick up bobble heads and such. I should be able to switch to a string pack without too much trouble.
People who bring in huge bags filled with all kinds of gear is why we can’t have nice things.
