Hudson River path in Manhattan is surprisingly nice and scenic.
If you leave the path at around 160th street and climb up to Washington Heights and bike over the George Washington Bridge, there's another bike path on the west side of the Hudson going north. Part of it is a road but it has little traffic, there are some short steep hills.
looking back south
Further north in Rockland County, sections along the Hudson have nice paths. Two miles north of Nyack is a 6 mile long bike path by the Hudson in a state park, the first section is flat
then it goes on small hills above. All photos mine