about a year ago they opened up the top of the Rockefeller Centre for sightseeing... makes up for the loss of the World Trade Centre's viewing platform, though it's further north, and closer to Central Park than the Empire State Building too. I mention it because it may not be in all the guidebooks yet, but the view is outstanding.
TKTS offices offer half-price tickets to shows that evening... the one in Times Sq tends to have big queues but the one at South St Seaport, down near the tip of Manhattan, is usually much quicker if you're in the neighbourhood.