Best way to get to New York City from Philadelphia for a day trip?

Next Saturday my girlfriend and I want to spend the day in New York City. I am currently in West Chester, PA which is about 25 miles west of Philadelphia. We are wanting to go up early and spend the day going around and then come back in the evening or late evening. What are my best options for getting into NYC? I've looked at Amtrak but its nearly $200 for both of us round trip. I've considered driving but I hear that may be a bad idea. Any suggestions on the best way to go about this?