On a recent trip to photograph the Tarrytown lighthouse, when I finished, I went back across the Tappan Zee bridge (there's a good view of the Tarrytown light just as you go onto the bridge headed west) and decided to take the short run up to Stony Point anticipating that the lighthouse would again be accessible since it was Labor Day weekend.  It was...

There are several routes out to the lighthouse... I would recommend going to the left and then up over the hill to the stone platform for the view on the top left below.  There are also commanding views of the Hudson River to the south from beside the lighthouse.