Not the colourful, famous Damnoen Saduak floating market, which I personally think is a terrible tourist trap and several hours drive from Bangkok, but Taling Chan Floating Market, in Chak Pra, just over the river from Bangkok. It was set up as a floating market in late 1980’s using five wooden barges; however, was later upgraded with 6 more metal barges all connected. You will not find souvenirs here, rather it is all about food and drink. This is the perfect place to spend a long lunch of fresh fish, prawns, som tum and other foods washed down with several cold beers.
Best to arrive early in the day, by lunch they are often full, find a small table and then order, a waiter or waitress will look after you, and run to the floating boats that will cook your food fresh as you order it. The prices are very friendly and you will not see many tourists here, mainly just local thai here for a meal.
You can drive there, and park nearby, but a much more interesting prospect is to catch a long tailed boat, just negotiate with any boat along the Chao Praya River, though BTW Sky Train Station Saphan Taxsin is easy place to do this from.
Along the way you will pass by many traditional wooden houses, other river users and a temple that allows tourists to feed their catfish. You may also wish to arrange with the boat driver to stop at the famous Thai Royal Boat shed. Your long tailed boat driver will then return you to the pier where you can catch the Skytrain home. A perfect way to spend a few hours in Bangkok!