Well, safety is the top priority at Hershey Park, and that includes making sure no one brings in any, ahem, "unauthorized" items. Don't worry, it's not as scary as it sounds, but you do need to know what you can and can't bring in. For example, backpacks are a no-go, but small purses and fanny packs are okay.
Now, you might be thinking, "But what about all my essentials: sunscreen, water, snacks...?" Don't worry, you can still bring those things, just make sure they're in a clear plastic bag or a small container. And, fun fact: Hershey Park offers free water at various stations throughout the park, so you can stay hydrated without breaking the bank (or carrying a huge water bottle).