I've been in late January, it was absolutely freezing, but still great. Warm coat, hats, gloves etc. a must.
For views, top of the rock and one world observatory. Do one at daytime, one at sunset. You get a better view of the Empire State Building than going up it.
The highline won't be so great that time of year, but that's not to say don't do it.
Can totally recommend walking the bridge from Brooklyn to Manhattan at dawn, but if not, go to Brooklyn to explore Dumbo and eat pizza and walk back over it at night.
Hillstone restaurant - great burgers and steaks and nice Californian wine.
The meatball shop was good for lunch.
Really enjoyed the Met.
The tenement museum is interesting if you like history, but need to book.
I haven't been to Edge as it opened since my last visit, but a friend enjoyed it recently.
Plan your days in location clumps so you're not cross crossing the city all day long. Walk as much as you can, remember to look up!