Some others like Amphawa are still real but crowded to such extent it's not fun anymore. Even walking along the river banks feels like being caught in the subway on rush hour... On the other hand, you can find less crowded floating markets but calling them this is a bit of an overstatement to be honest, as six boats parked next to a pier is more of a riverside markets, but everyone knows it wouldn't attract as many visitors.
Then there is Taling Chan floating market, with just enough boats to be called a floating market and the right amount of people to still be enjoyable. Located 12 km outside Bangkok it is still easily accessible without having to get on a tour or even having to wake up at 6 am! If you don't have your own transportation you can ride the BTS until the last station 'Wongwian Yai' then get a taxi for about 150 baht.
The entrance of the market is not all that impressive with that green plastic roof sheltering mostly plants and gardening stuff but beyond is the real market with all kinds or sweets, fruits and fun snacks you should really try. Travelling without eating is missing half the pleasure.
Past a small bridge, you can't miss all the wooden boats moored along the riverbank, most of them cooking huge prawns on small barbecues, blue crabs, large fish, some others preparing the famous Som Tum for the many dinners sitting at the usual wooden platforms and low tables. Eating seafood at the floating market is a great weekend attraction for Thai people and if you arrive past eleven in the morning, restaurantsare already running at full capacity and most tables in the central area are occupied. Continue past the first group of restaurants and the crowd starts to thin out.
If you don't want to eat on the floor, you will find some restaurants with plastic tables and chairs near the railway bridge. Restaurants usually have a small illustrated menu so ordering is not too challenging. Worst-case scenario just point at what you'd like on your neighbour's table. Four of five of these giant prawns cost around 200 baht and taste great!
Eating by the river is such a fun experience, you should really try it. Surprisingly while the cook are grilling fishes on their many little boats, hundreds of huge catfish are permanently swarming the waters around the canal. Obviously those massive fish are well aware of their bad taste and their popularity with tourists who just love to feed them colourful pellets all day long. To add to the exotic atmosphere, a group of Thai traditional musicians play for few hours adding a real nice touch to the entire experience.
Just next to the dining area an incessant stream of longtail boats add a local touch to the whole experience. You can actually ride on of these boats for a quick tour around the neighbouring canals, or 'klongs' as they are called here. It's not very impressive but is a fun addition to your daytrip, you will pass by temples, witness the life of people living by the water, and you will without a doubt spot at least one or two giant monitors.
On the way out, end your little day trip at the open air foot massage set under the trees, it is cheap (200 baht for an hour) and perfect for a little snooze if you don't mind the permanent chatting of the masseuses who seem to be catching up on this week's gossip! Going at around 10:00 in the morning mean you avoid the heat and the crowd. Be sure to bring cash as no one accepts credit cards.