The Financial District is small. That's the thing most visitors get wrong about it. People plan a whole day for Wall Street and another day for the Seaport and another for the 9/11 Memorial, and then they find out all three sit inside a fifteen-minute walk of each other.
We're on John Street, roughly in the middle of it. Wall Street is two blocks south. The Seaport is a few blocks east, downhill toward the water. The World Trade Center campus is west, past Broadway. So this list is ordered the way we'd actually tell you to walk it, not the way a top-ten list usually goes.
A few honest notes before you start. The streets down here are old and crooked, so your phone's walking estimate is usually off by a minute or two in either direction. Weekday lunchtime (roughly noon to 2) is the one time the sidewalks near Broad and Wall get genuinely crowded, and weekends are the opposite: quiet, easy, good for photos. And almost everything on this list is free, which is not something you can say about many parts of Manhattan.