Don´t waste your precious time booking a half-day excursion to Liberty Island. Instead, take the Staten Island Ferry ( yes, the same one that Melanie Griffith took everyday to conmmute in "Working Girls"). It is free and the views are absolutely breathtaking. Total duration: 2 hours aprox. Train: South Ferry.
The ferry departs from Battery Park. Here you can see the views chicalista is talking about. I'd take her tip ;)
Liberty statue, lower manhattan and the ferry itself. it seems to be crowded but don't worry, there's plenty of seats.
At the southernmost end of Battery Park, there's Battery Gardens, an outdoor bar. Have a drink and enjoy the great views of the harbor before or after your trip on the ferry.