Newly re-opened since my last visit to New York, the Top of the Rock is the perfect place from which to view the Manhattan skyline. For a start, you actually get to look AT the Empire State Building rather than looking OUT from it. The top level balcony is also open (the Empire State is enclosed) so it's a slightly less claustrophobic experience.
The Top of the Rock is part of the Rockefeller Center, which also houses Radio City, NBC (including the Saturday Night Live studios) and the Rainbow Room. It's also famous for its Christmas Tree and the Ice Rink (which was still there in March).
I have to confess that we did the Tourist Thing - in this case posed for our very own version of the famous Girder Photo. Although our feet were fairly close to the ground. The results are currently displayed on a fridge magnet near me ;-)