Since Bangkok is located at the very end of the mighty Chao Phraya River that makes its end at the gulf of Thailand, this means it is the delta where there are canals and other small rivers connect communities in a few provinces around this gigantic city. In the past, road system was not as good so people traveled, traded by boats, even lived on the boats. Nowadays, rivers and canals are still used but not as much as before anymore. However, many communities still live in the lifestyle similar to their forefathers; at the same time adjust themselves to the modern world too.
Many of them open to tourism and promote as the floating markets. The very popular one would be Damnern Sadouk floating market which can be very touristy. (You will see more foreigners there.) There are others that are known probably among Thai tourists more. One of them is Amphawa floating market. Since it is not far from Bangkok so it is possible to make a day trip to there and explore the place, or if there is more time it is also nice to stay overnight, relax and chill out, at one of the old wooden houses (either guesthouses or homestays) at the side of the river. This could be memorable. The trip can be done both by joining the tour which is more convenient or use public transport to get to there. The 2 main points to get the bus or minivan are at Southern bus terminal (There are 3 main bus terminals in Bangkok.) or at Victory monument and that costs only around 3 usd per person and takes around 1.5 hour maximum (well, depending on the traffic in the city) to Amphawa. There is a big sign at the entrance gate welcoming everyone to the community. Visitors will be amazed by how colorful and lively the place is. Lots of boats filled with goods and food. The long tail boats are for passengers who want to tour along the canal. There are also a lot of souvenirs and handicraft shops in those wooden houses. Either walking along the 2 sides of the canal or into the smaller alleys is a very fun activity to do there. Inexpensive food, drinks, ice cream, and Thai desserts there plentiful. Going there with a tour guide or local friends (who can speak English) can be the plus in terms of understanding the culture and things more in depth since there are so many little details that they can explain to you. Not just only day boat tour, but night boat tour to see the fireflies are available for those who choose to stay overnight too. Or, if once wants to explore further there are bikes for rent to see the farms and beautiful rural areas as well.