Years ago, before we had kids, Hershey didn't charge admission to the park. They'd operate on a ride ticket system, so my husband and I would stop for some Hershey's hot cocoa and cookies and just walk around the park looking at all the lights without riding anything (for free).
If you go, one of the overlooked neat treats is the dancing lights on the bridges. Every so often music will start (such as a selection from Trans Siberian Orchestra) and thousands of dancing lights will accompany. Not quite the caliber of Osbourne's Festival of Lights at Disney World, but still very neat (and MUCH less crowded).
This year, we had the privilege of going on a warmer day and a school night so it was very empty. Definitely recommend! Merry Christmas!