Ancient coins from by-gone Thai and Burmese kingdoms. Perhaps you can find a rare one?
Carved and ceramic images of the Buddha standing, sitting and lying are sold in every market. These images are revered. Made in Thailand they require special licence's if they are to be taken out of the country.
These guitars are only about 10 cm long. Miniature Western and Thai instruments make perfect souvenirs. Could be a little difficult to play though!