My sister lives in the general area, so we have gone to the park several times. There are a few places near the entrance where you can park your car, walk in and get free accesss, so be sure to drive around a bit. Don't do that on the day you have a huge picnic basket, though. It won't be fun.
The fireworks are always fun, and be sure to climb the mountain if you are able. The view is not to be missed.
The visitor section at the top is lame, after the long walk, I think people deserve more than a dull snack bar and two sets of bathrooms (one of which is ALWAYS closed).
Go at least once.