Grand Central is big and beautiful and it also periodically hosts free cultural events - poets at typewriters custom-composing poems on demand - or Alvin Ailey dancers performing in Artist, Nick Cave's horse suits!
I love sneaking over here in the middle of the day to see a movie - they always have the best selection of films. I recently saw Pina in 3D and The Kid with a Bike here, and another time I saw Bela Fleck play the banjo after a screening of Throw Down Your Heart.
The print graphic designer´s heaven. All sorts, sizes and weights of paper, shaped in forms i have not seen before (jup you don´t get that in Austria). Packaging material, adhesive letters and everything else you just want to have. Find all sorts of pens and versions of Moleskines. If you like childrens books and/or illustration, also check out the great store on the other side of the street (Cupcake Café).
A great way to escape the city...Especially amazing in Spring when the cherry blossoms are out. I love the Japanese pond, the famous NYers walk with plates for Arthur Miller or Lee Krasner and of course the green house.