If you buy your Statue of Liberty tickets online (!!!!), and you get up and get out there early, you could have the 2nd half of the afternoon free. I wouldn't plan on it, but you might end up finished before you realized.
We went a few years ago, and I bought the tickets online. We saved ourselves easily 2 hours by having done so. I don't know if more people have wised up, but I was amazed. The security guard on the line said, "Who was the genius who bought the tickets ahead of time?"
Glad to hear all the corrections about not needing a car in Seattle. That's way cool!
(Most other cities I've been to, even Mpls, w/ its decent-ish buses, you can only get to a few limited places w/ the bus.)