I think I had about five conversations before I got to Thailand that went roughly as follows:
"What days will you be in Bangkok?"
[response]
"Oh, then you can/have to go to Chatuchak."
So clearly I had to go to Chatuchak.

Chatuchak, besides being absurdly fun to say, is a gigantic weekend market on the north side of Bangkok. People say you can get anything at Chatuchak. I would like to amend this to say you can get anything at Chatuchak provided you can manage not to get lost while there. I myself ended up in front of the same stall three times (from different directions!) because I kept getting turned around in the myriad aisles and alleyways. The market is so large that there are maps made (and sold) of it. There are 27 numbered covered sections, each featuring through-alleys and grids of narrow aisles such as the one pictured here. There are also three banks, a police station, a management office, 8 sets of restrooms, a clock tower, a first aid station, and three gates leading in and out of the market grounds. It's hard to tell from inside, but on one map I obtained the grounds appear to cover the rough equivalent of three or four city blocks. Across the street is a separate meat and vegetable market that operates every day of the week.
As you can guess from the picture, it's crowded, with both tourists and locals. Most aisles (but not all) can just about accommodate two people passing without one having to turn, but since people are often stopping to look at a vendor's wares, things get congested pretty easily. Which, combined with the limited air circulation, makes it, guess what, HOT! A couple of vendors, particularly among the pet stalls (puppies, kittens, birds, fish, various rodentia, and some very small mammal of which photographs were not allowed (there's a good sign something's above board...)), had air conditioning which they used behind closed glass-walled stalls and doors, so that they looked like they were in their own little aquariums.

There's also food aplenty. These were some of the snacks on display at the main market (not the one across the street). Although there are a few food vendors deep in the bowels of Chatuchak, most, and particularly the fresh-fruit and drinks vendors, line the main walk between the numbered sections (the main walk functions a bit like a promenade, where you can take things in casually with a bit of a breeze -- albeit exposed to the blazing sun -- before darting inside the maze to do actual shopping).

Remember when I said you could get anything at Chatuchak? Seriously. You could outfit a rancher (or a skate teen, cheerleader, working woman, etc) for under US$15: plenty of denim shirts, jeans, all kinds of leather anything, including big Texas belt buckles and boots... country bluegrass music... that's right, that's a Thai cowboy playing spoons. I bet he bought his entire outfit there. If I'd looked hard enough I'm sure I could've found spurs, too.