The Gray Line Hop on/Hop off double decker bus tour is a great alternative to a regular bus tour. It is very flexible in that you can get off at any of the stops and stay as long as you want. The day pass cover 5 different routes including a Brooklyn loop that operates seasonaly, and a night loop that hits the restaurants and evening entertainment venues.
The downtown loops, start at the Empire State Building on 34th Street and has stops at Greenwich Village, Soho, Chinatown, Tribeca, and Battery Park, for excursions to the Statue of Liberty and Ellis Island.
The midtown loop makes stops at Central Park, Rockefellar Centre, Times Square, the New York Public Library and Grand Central Terminal. If you are staying in midtown Manhattan, you can save this loop until last if you have time, as most of these stops are within walking distance to the midtown hotels.
The uptown loop starts at Central Park and goes completely around Central Park, as far north as 125th street in Harlem. The highlights of this loops are the many museums that New York has.
This tour gives an excellent overview of Manhattan, and the guides provided us with wonderful tips on everything from how to use the subway, to popular restarants and nightsclubs, and those which should be avoided.
I would not do the tour again, as I now have a better concept of how to get around on my own, but it was definitly worthwhile for a first time visitor.