I live in NY and I still haven't been to Ground Zero and the last time I was at the Statue of Liberty was a 1st grade field trip! However, I do plan on taking my family there soon. You can buy the Liberty Island/Ellis Island tix in advance online. Also, consider the Freedom Tower as well (the building that replaced the Twin Towers). Check Groupon for discounts, the elevator ride to the top is an attraction in itself. There are lots of walking tours that are inexpensive and can cover a lot of ground. You may want to take a look at the City Pass to see if it can save any money. It really depends on what you want to see and how much time you have. Tons of awesome museums too (Natural History, The Met, The Intrepid) but again, those take up a lot of time - we spent at least 6 hours in the Museum of Natural History and still didn't see everything!