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).
As Jim Jarmusch put it in the documentary Blank City, the address is roughly between Bowery, Avenue B, 14th Street and Houston. This area of the city is steeped in art, film, and music history; so many hugely influential artists, film makers, and musicians still live and work here. For such a small area, I think it's had more concentrated influence on contemporary art and culture than anywhere else.
I love that there's a museum dedicated to American art, founded at a time when American artists were underappreciated. The exhibitions are curated with a broad but discerning eye, and the architecture is spectacular. In my experience it's typically much quieter than the obvious choices like the Met and the MoMA - although both are remarkable, there's something I love about the scale and style of the Whitney.