I always book flights to Thailand so I can squeeze in two weekends over the course of my stay, because chatuchak market or better known as JJ market to the locals is only open on weekends. It is a massive wholesale market selling clothes to food to crockery over 27 acres which is 15,000 stalls. This is the place that other shops come to get their wholesale goods. So if you're looking to get souvenirs this is the place to go, they have all your elephant shaped goods and silk scarves as well as a bunch of more unique crafts. You'll need lots of cash. Start early, shops tend to open at 10am, so you can beat as much of the heat as possible, and if you, like me are lugging around a reluctant shopper, there are massage places pretty much everywhere.
Also don't eat, there's a wealth of street food to sample. Must tries include the coconut ice-cream that comes in a half coconut shell topped with you're choice of sticky rice, shaved coconut, peanuts and other stuff I never pick; Thai iced tea and arguably the best fried chicken shop. The papaya salad was a bit disappointing but the fried chicken is better than KFC.
Nothing like Bangkok like the food, Thai people love Japanese food (as do I) and they have some of the freshest seafood around, so it makes for great sushi trains. Also the pastry in the top left is a croclair - a donut/croissant/eclair hybrid, it was amazing. Food courts in Thailand are a must visit. The Central Embassy shopping centre, one of those fancy high end shopping malls has a basement food court that's set up like a street food centre but cleaner, and is definitely worth a visit. They had amazing pad Thai, and my favourite roti with banana and condensed milk. Also bottom photos are of the food at Coffee Bean by Dao, they have amazing cakes and a mix of Western and Thai food, there's one in Siam Paragon.
Also worth a visit is the Erawan Tea Rooms, it had a special when we went, a mix of traditional Thai snacks and a western afternoon tea stuffs.
The best thing about shopping centres in Thailand is that they are all linked along the train line, but I reckon you'd be hard pushed to visit more than one in a day. My favourite (because I'm a cheapskate) is Platinum Mall, it's a wholesale women's fashion mall. Most items have prices written up, but the more you buy the better the discount. I was too busy shopping to take photos.
So fabric shopping in Thailand is centralised to China Town, and Little India which are both connected. Little India tends to be more sari type stuff, whereas China Town has good basics and quilting fabric and haberdashery, all focused along Sampeng Lane. Jill over at bkkquilt has put together a good map of the area, and even managed to get some sneaky photos in (as most all the fabric stalls have a no photography rules). The fabric stalls are squished between a lot of handbag, Disney product, soft toy and jewellery stores. Also mostly wholesale. If you want a hair tie, you'll end up with a pack of twenty instead, but basically for the same price.
I managed to get a pretty good haul in Sampeng. I got a bunch of quilting cottons/voiles for 89 baht/meter (~$3AUD/meter) and some polyblend suiting fabric. The haberdashery cost me around $11 AUD for a dozen 22 inch invisible zippers and a half dozen different spools of thread.