One of my favourite weekend escapes from Bangkok is Amphawa Floating Market in Samut Songkhram. It is a popular place for Thai people though an increasing large number of foreign tourists are finding their way there. It only takes place at the weekend and also only from about 4 p.m. onwards. If you have the time it is best to spend the night at a homestay as the place is nearly deserted during the morning. I like exploring the area on a bicycle or renting a long-tailed boat.
Map showing location of Amphawa Floating Market in Samut Songkhram: