I once spent a thunderstorm riddled night at Codorus in a tent. I taught my sons how to determine approximately how far away the lightning was by counting the seconds between the flash and the thunder.
Drive into Hanover and take the Utz Snacks Factory tour.
A visit to the Harley-Davidson plant in York, PA is worth the ride. However, please be advised that there are age restrictions and dress requirements for the factory tour.
York Facility Factory Tours | Harley-Davidson USA
Too bad you're a little early to enjoy the swimming pool. Codorus is one the few PA state parks that has one.
Enjoy your stay!