My brother and I took a short trip to look for some souvenirs at the Chatuchak Weekend Market, which is a place I visit every time I go home. It is one of those places that is popular amongst tourists AND locals, so it's not just a tourist trap and it's worth a visit!
How to get there: The subway MRT (Chatuchak Park Station) and the skytrain BTS (Mo Chit Station) both go right to the market. When I leave, I don't like to take taxi from the market, cuz it's kind of a pain and there will be scammer taxis who are waiting for foreigners there.
My Chatuchak Tip: It's huge. And not everything will be of interest to you. Luckily, things are divided into departments so think about what you want to explore (food, clothing, home stuff, plants, pets...) and find out where that section is (I'm sure there's a map somewhere). It'll be hot, but don't drink too much water cuz bathroom isn't easy to find there especially if you're deep in the middle of the market!