HersheyPark now owns me.

[Undertoad]

You can go in the summer if you have a flexible schedule. Time your trip around weekdays where the previous weekend was sunny and where the rain is supposed to end mid-afternoon. Enter the park just as the rain is ending. weather.com has hourly forecasts if you really want to get precise. The rain will scare off the long-distance all-day busses, the good weekend will have satisfied the big crowds, and you'll have lines a quarter as long as they would be because the rides run at the same speed anyway.

When I lived ten minutes away, the strategy was to go on days when there were off-and-on thunderstorms predicted... the storms would chase everyone out and leave us with two-three hours of line-free nighttime riding. You might have to wait out a passing storm which was fine.