You did what this weekend? - "Oh, you know, I took a Kayak out from 63rd St, paddled my way down the Hudson, took a little spin around Ellis Island, and shot a few pictures of the Statue of Liberty." Not only is Kayaking great exercise, easy to pick up, and a lot of fun, it's also one of the best answers that New York has to offer to the question "Do anything interesting this weekend?"
The Downtown Boathouse
From May 15th to October 15th the New York City Downtown Boathouse has free walk up kayaking. The only requirement is that you know how to swim. All you need to do is show up with a towel and a bathing suit at one of their three locations (Pier 26 @ Canal St, Pier 66A @ 26th St, or Riverside park at 72nd St). They also organize free 3 hour tours (fear not Gilligan!) on a first come first serve basis.
http://www.downtownboathouse.org/programs.html
The Manhattan Kayak Company
For those who don't want to queue with the hoi poloi for a kayack Manhattan Kayack Company offers private instruction and a wide selection of tours ranging from $55 - $250. Some of their tours are considerably more adventuresome than those at the Downtown Boat House including an eight hour circumnavigation of Manhattan and a ten hour trip to Riker's Island.