We didn't hit the Circle Line Tour on this trip, but we have done it in the past and it is FANTASTIC! It's a relaxing, and very scenic way to easily circumnavigate Manhattan Island in it's entirety.
If you are a first-timer, or plan on being in New York for a while, I'd really recommend the Circle Line Tour as a starting point for you to relax and get your bearings about yourself in relation to most of the major landmarks available to you. Get comfortable on board with a refillable souvenir cup, as you listen to the super helpful narration. Don't forget your camera, because you'll have loads of unique vantage points from which to capture priceless memories!
A block north of Circle Line, you'll find the USS Intrepid Air & Space Museum. This US Naval Aircraft Carrier is a PHENOMENAL way to spend an afternoon!