Benchawongse! The uniquely Thai porcelain hand-painted in five colors. Various corners of Chatuchak have stalls selling benchawongse, though the quality is mixed. Narai Phand, formerly the Thai Industry Shop, across the street from Central World Plaza on Ratchadamri Road has the best selection in the kingdom, all with the original carrying cases designed specifically to hold and transport 'em. (I once brought a very large vase home in checked baggage without incident.)
Apart from that, piles of Jim Thompson silks. For 1100 baht ($30), you can get neckties that are as high quality as Ferraggamo at 1/4 the price.