In NYC:
-WALK!
Walk in the beautiful park along the Hudson between Chambers Street and the Battery
Explore Central Park: It's HUGE. Look at a map before hand and maybe see: The Mall and Bethesda Fountain, The Belvedere, The Conservatory Water, The Zoo, ...and The Rambles* .
Along the Highline
Take the aerial tram to Roosevelt Island and walk at least to the Roosevelt Memorial at the tip, if not the entire perimeter.
Wander through the West Village, staying off the Avenues (which are loud and very commercial) favoring the tree-lined side streets. Sit at outside at a cafe or restaurant and watch the crowd
-Take at Tour of the United Nations
-Go to the Metropolitan and see at least the Temple of Dendur, the Chinese courtyard, The American Court, The Medieval Court, The Greek and Roman atrium, . It's also huge: just wander. Stop at the Petrie Court Cafe for a coffee: It's nothing special except you can see Cleopatra's Needle in Central Park through the windows.
-Take the [free!]Staten Island Ferry round trip just for the lark of being out in the upper harbor. . Be sure to time it so your ferry is on the water EXACTLY at sunset or a few minutes before.
Give him several hours on his own mid visit to decompress and have some private time (if he wants)!
*The Rambles is also a fabled after-dark gay cruising area, which at that hour is not terribly safe at all, or really that good a cruising spot anyway.