1). Brave the crowds on 5th Avenue and the Rockerfeller Center. The Christmas decorations and window displays (particularly at Bergdorf Goodman pictured) are spectacular and you have to go and try on at least one pair of insanely expensive shoes. If you're brave (I wasn't) go in the morning when there aren't queues and go ice skating.
2). Eat at Five Guys. Traditional style burger joint, there are several as they are now a national chain but still feel like a one off place. Nothing flashy but the burgers and skin-on hand cut fries are incredible. Make sure to get ALL the toppings and I recommend getting takeaway and eating in private. It gets messy.
3). Visit the Guggenheim. NYC is full of amazing galleries and museums but to me the Guggenheim is the best. It's a stunning piece of architecture and such a clever layout. It's size and format means the exhibition is really well curated and entirely manageable. No art over-exposure or sore feet. I've been twice and loved it both times. Even non art lovers will be able to handle this one.
4). Go to Brooklyn, especially up to Williamsburg. If I went again I'd spend most of my time over there. It's too cool. It's so nice to get away from the noise of the city too. You must go to Beacons Closet (both of them) and Buffalo Exchange thrift shops, great pickings at fair prices just give yourself plenty of time and energy. Check out all the little shops you pass as well, especially on 5th Avenue in Brooklyn.
5). Ok I'm going to cheat on my 5 things and put two together. Go to Grand Central Station and just marvel at the ceiling. It's so pretty, plus you can stand there and think about all the movies that use it as a place to lose people (so if you're travelling with someone you want to ditch go here and run off). Finally, obviously you have to go to Central Park even (or especially) if it's snowing. It's a great place to people watch and see lots of pampered NY dogs and crazy dog walkers walking 12 dogs at once.
Now I'm really going to cheat and share just a couple of quirky shops you should checkout.
I was really lucky to get some great recommendations on where to go. Guide books are one thing but it means a whole lot more fun when you can get some real life tips from some like-minded people. So with that in mind please share your favourite places, shops, things to do and eateries in the comments.