Probably the easiest to get to in terms of distance from Bangkok, Bang Khu Wiang Floating Market is probably the most difficult to deal with because of its opening hours - 04:00 and 07:00. Sellers bring natural goods (vegetables, etc.) and it is also possible to see monks rowing boats to collect alms! Not at all commercialized, this is probably one of the few places you can see such a traditional way of life around Bangkok.
Details: The market opens between 04:00 and 07:00
How to get there: Probably the easiest way to get there is by boat down the river. You will arrive in Bang Kruai and from there it's a 10-minute boat journey to the market. There are a limited number of tours so otherwise a taxi might be your best bet. At that hour in the morning, chasing around after buses is not recommended!