Catch a BTS to Saphan Taksin station, come down to street level, turn away from the river and walk to main street, Charoen Krung rd. Turn left and just walk, and walk and walk. A little way down the road you will come to Chinatown however there is so much to see on the way. Go down alleyways, stop at markets, have a coffee etc. Chinatown at dusk is the best time, bustling and frenetic. Fantastic restaurants and street vendors. My son and I walked around for hours and found the area fascinating.
We chose a restaurant for dinner and had the very tasty, famous, oyster omeletes. And cold beer of course!
If you are not hungry in Chinatown, and are heading back to the BTS, just to the side of the station, is a small road that leads to the Bangrak Bazaar which has a lot of ladies garments for sale as well as a great variety of street food. Cheap cheap and very tasty and authentic. Many locals eating here.