One of my favourite fairs is currently on in Bangkok at the Impact arena. The OTOP show has hundreds of stalls selling goodies from all over Thailand, plenty of food to sample and good gifts to take home.
More info here: impact.co.th/index.php/…otop
My tip, get there early and head straight to the 5 star curry puff man from Sukothai, he is extremely busy so you have to place an order and collect a few hours later. They are the best curry puffs I have ever had in Thailand.