We had one of those 'free' ferry tickets. Sadly they give priority to paying customers first and us freebies didn't get on. Waste of a couple of hours getting to terminal and queueing.
Bloody hell, just looked up the price of the hop on hop off bus. £50 pp.
Walking from your hotel to World Trade Centre is over 3 miles. I would suggest taking a map of Manhatten and pinning places you wish to see and make two routes. Upper and lower Manhatten
For instance, walk to Grand Central...
Have you booked a hotel?
Check the hop on hop off bus route.
Statue. Do the free Staten Island Ferry. Takes you close to see the statue.
Empire State Building is a few hours. Book just to make sure.
Ground Zero is something you can walk around.