Well, if you're in Soho, there's pretty much all the shops you need right there and you can spend a good day just mooching around, window and actual shopping. You have to go to the Apple store and the Prada store down there because they're retail experiences.
Greenwich Village is also good for shops and pretending you're in Sex and the City.
Also you're not too far from the Statton Island Ferry which is the only thing you need to do to see the Statue of Liberty.
Go over to Brooklyn for a nice lunch by the water and the Woody Allen "Manhattan" view.
Uptown for shopping there's Bloomingdales and Saks of course (also a v g Bloomies in Soho) and you head further north to Barneys and go to Calvin Klein and Ralph Lauren.
If you want a spa treatment go to Bliss.
If you're going at the weekend, go to Central Park and hire bikes or roller blades on a Sunday.
Not too up on what's on at the theatre at the mo, but do try to go to a show.
Couple of good restaurants are Union Square Cafe and Gotham Bar and Grill.
Sorry, have just seen that you've been before so ignore sight-seeing bits in this post. But you should defo stay downtown if you haven't before: the Mercer, Soho Grand, 60 Thompson - all very cool