One of the only things I enjoy about taking the subway these days is seeing the defaced ads on the L train. Whether it's random chance, high schoolers or street artists the result is always good.
Occasionally I'll look at the Manhattan skyline and wonder what it was like in 1931 when people around the world were saying, "Holy shit, did you hear what they just built in New York?" Just get up there and trip out about humanity. And be sure to keep an eye out for the unmarked locked door guarding a set of stairs leading to a hidden terrace that was originally designed as a fucking docking station for zeppelins.
Grand Central is big and beautiful and it also periodically hosts free cultural events - poets at typewriters custom-composing poems on demand - or Alvin Ailey dancers performing in Artist, Nick Cave's horse suits!