Here, straight from the Fashion's Night Out Website, is my itinerary for the whole night. As you can see, most of the action is clustered around the Midtown, Meatpacking District, and Soho areas. Expect the crowd to be crazy at the Barneys, Bergdorfs, Jeffery, and Opening Cermony events (last year, my friend and I waited 45 minutes on line to get into OC before calling it a night and moving on).
I will definitely be getting to Barneys early to start my night (with a quick stop at Dolce & Gabbana before and Calvin Klein after), then on to the Plaza to meet the young designers from the CFDA Incubator project and Douglas Hannant. After that it's fair game downtown. I've heard Billy Reid's bash was a success last year, and I'm sure Oak's event will be full of gothic hipsters and fashionistas.
If you are looking to spend the duration of your evening in one location, I would suggest one of the big department stores. I would say Bergdorfs (with appearances by Tom Form, Thom Browne, Simon Spurr, and Victoria Beckham) and Barneys (Mary-Kate & Ashley, Rag & Bone, Shipley & Halmos) are your best bets.
Both Jil Sander and GANT will be debuting new lines, and Moncler will finally be opening their Prince St. doors.
Opening Ceremony will most likely have their "limited edition" merchandise from designers such as Alexander Wang, Rodarte, and Proenza Schouler available past the event, so don't fret if you don't make it down to the ACE hotel in time to beat the crowds.
All this said, I attended last year without a plan (and it wasn't for lack of trying) and still had a great time. This year seems offers a more shopping driven experience with stores offering more exclusive "one-night-only" product as well as complete store openings and the usual shopping incentives.
Have a great night!