5th Avenue is one of the most famous streets in the world, and along with it's attractions (e.g. Empire State Building, Rockefeller Centre, Flatiron and the Guggenheim) it's famous for it's shopping.
Do your research on shop positioning before you set off, as the street goes for kilometres! The end closest to Central Park is pretty pricey with a variety of shops spread out over the avenue. Don't forget to drop by Saks Fifth Avenue while you are there!