Catch the historic Circle Line at West 42nd Street and 12th Avenue for this surprisingly worthwhile 3-hour tour. The tour usually travels counter clockwise around Manhattan Island, but occasionally will go the other way. Either way, it covers the entire waterfronts of the Hudson and East Rivers and the skyline of New York Harbour.
Perhaps I was lucky, but the day I did this tour the narrator was outstanding in his knowledge of the island and its history. It was an educational experience.