...should go to Roosevelt Island?
It is hardly ever mentioned in guide books or by people who have been there before, but it is fabulous! I'd visited 3 times before I stumbled across it. not a stealth boast
You get to go over the Hudson River on a tram/cable car for the price of a subway journey, then there's a lovely visitor's centre with hot chocolate. There's a ruined old sanatorium, a memorial to Roosevelt, a lighthouse and a free bus that takes you round the island!
And it's so much quieter than Manhattan so you can have a little break from the crowds before you enter back into the fray.
And the grocery shops are cheaper. 