One of my favourite riverside markets in Bangkok is Khlong Lat Mayom Floating Market on the Western outskirts of the city. It is in the same district as the more famous Taling Chan Floating Market. It is best to go early as it gets very crowded as the morning progresses. Go hungry as there is a lot of delicious food to taste and eat. You will need at least half a day to explore as you can also go on a boat trip. Like other riverside markets, it is only open at the weekends and public holidays from 8 a.m. to 5 p.m.
Map showing location of Lat Mayom Floating Market in Bangkok:
GPS Coordinates: 13.761904, 100.415616