This is the island of Khung Bang Krachao. The NY Times once called it “the Green Lungs of Bangkok.” It's about 20 minutes from downtown on the sky train (45 minutes in a cab) and it's a wild, lush, floating island, in a curve of the Chao Phraya. You can stay at a place called The Bangkok Tree House for approximately $130 per night and book a bike tour with local expat Paul Mueller. The bike tour is $30/3 hours and is exceptional.
Read more about it here: http://saltwatercureseverything.wordpress.com/2014/04/17/pedaling-through-a-pigs-stomach/