Right on the Hudson River, Rockwood Hall is the site of a mansion once owned by William Rockefeller. Unfortunately, very little remains beyond the foundations of the house, which are quite impressive in their own right. It has great views of the river (e.g. towards the Tappan Zee Bridge) and the trails connect up to the Old Croton Aqueduct trail. Turn left and you can walk up to the Croton Dam; turn right and you go into the Rockefeller Preserve. This summer evening the light was gorgeous.