I agree on the B&H. Its like an adults electrical willy wonkas. They have overhead conveyor belts which transports your purchases to the exit where its bagged up and waiting for you. The staff are all orthodox jews so the shop isn't open on saturdays but it is on sunday. They are very helpful and knowledgeable and I've never experienced any hard sell. I love it. Oh and had a fair few bargains there too. Although, Im going next week and Im not expecting too much this time as the dollar is far stronger than my last few visits.
The apple store is one to visit (if you into apple products) on 5th ave opposite abercrombe and fitch (which is also worth a visit ;-)). Its the flagship apple store and its huge.
Ellis Island is cheap to visit and very interesting.
Also the tenement museum tour is pretty good, on orchard street, lower east side on Manhattan.
Obviously, you must visit ground zero, its hard to believe the sheer scale of it until you see the size of the site. I was lucky enough to have been up to the top of the towers before 9/11 and to see the place now is unbelievable. The site is difficult to see properly due to fencing but there is a small church next to it used for rest and feeding and watering the rescue teams. Its free to have a look around. Theres loads of stuff to look at in there including photographs.
If you walk further south from ground zero, you'll walk through the financial district, wheres theres some amazing buildings and finally you will arrive at battery park which is quite peaceful and theres some nice views uptown.
Heres a nosey squirrel in battery park
The subway is superb and very cheap. Plenty of photo opportunities on and within the tunnels and trains.
Theres lots of outdoor and indoor flea markets all over manhattan. You can spend ages rooting through the goods. Or just wander around people watching and obviously some good photos to be got.
The Top of the Rock, imo, is better than the empire state. The queues are far less and the views are better as you get to see the empire state in your vista. Also nice to visit at night. But if you have time doing both is worth while.
Central park is huge and theres loads going on. The summer time is best for central park. But winter is still good. Its beautiful in the snow, although there isn't any there at the moment. The ice rink is a good photo subject.
Hope you have a good one. Oh and don't forget to register your details online for your visa before you travel. (new US security procedure). Link below for free registration. Some sites try and charge you.
https://esta.cbp.dhs.gov/esta/esta.html?_flowExecutionKey=_c92EC372F-D578-04B8-76F7-AAD614AA83DB_k7F0F13C1-CE5B-1BF3-DC5E-D9A83B7E0FEA