I first visited Lam Phaya Floating Market after attending the Tattoo Festival at Wat Bang Phra. Its definitely worth visiting but it is out of the way if you are coming from Bangkok. It has everything you would want from a riverside market; floating restaurants, boat trips to local temples, and a land based market. There is also a free museum.
Map showing location of Lam Phaya Floating Market in Nakhon Pathom Province: