If you're a Seinfeld fan you have to go to Tom's Restaurant! It's in upper Manhattan on 2880 Broadway! Sit down have some coffee and pie and you'll be good to go.... It's actually pretty small inside!... ....... ....... .......
There is a lot of horrific advice in this thread. Please for the love of God do not go to Staten Island or Atlantic City. AC isn't that close especially if you're only going to be here for 3 days and nobody goes to Staten Island. Also, Times Square is the worst part of NYC. Please do not hang out there. Go out in the West Village. Tons of bars, restaurants and energy. If you like comedy, reserve tickets to the Comedy Cellar shows. You never know who might show up--a lot of times big names come through unannounced. If you like R&B music, The Groove always has great bands to chill to. If you're into jazz, go to the legendary Village Vanguard. These are all within like a 5 minute walk from each other, you can't go wrong. If you want to see a show that's a little bit different and will blow your mind, go to Sleep No More in Chelsea. I promise you that if you do this, you will talk about nothing else following your trip.